07 Heavy metal removal from contaminated sludge for land application A review--打印

Heavy metal removal from contaminated sludge for land application:

A review

Sandhya Babel *,Dominica del Mundo Dacera

Environmental Technology Program,Sirindhorn International Institute of Technology (SIIT),Thammasat University,P.O.Box 22,

Pathumthani 12121,Thailand

Accepted 28September 2005Available online 17November 2005

Abstract

In recent years,various methods for heavy metal removal from sewage sludge have been extensively studied in order to minimize the prospective health risks of sludge during land application.In this paper,a comparative review and critical analysis of the application of chemical extraction,bioleaching,electroreclamation,and supercritical ?uid extraction (SFE),in removing heavy metals from contami-nated sludges is presented.Moreover,speciation studies,which can indicate ease of leachability of the di?erent forms of heavy metals in sludge,are also presented.Experimental studies revealed a broad range in metal extraction e?ciencies of the di?erent extraction tech-nologies.Acid treatment seemed to e?ectively remove Cd,attaining as much as 100%removal for some studies,as compared to biole-aching.SFE also gave higher removal e?ciency than bioleaching.Cr,Pb and Ni seemed to be also e?ectively removed by the acid treatment.For the removal of Cu,Mn and Zn,the bioleaching process seemed to be appropriate with maximum removal e?ciencies of 91%,93%and 96%for the three metals,respectively,and as high as 64%minimum removal e?ciency for Zn.The SFE process also gave good results for Cu,Mn and Zn removal.Electroreclamation exhibited better removal e?ciency for Mn,but is still inferior to acid treatment and bioleaching processes.For chemical extraction,because of the adverse impacts that can result from the use of inorganic acids and complexing agents,interest can be directed more toward utilizing organic acids as extracting agents because of their biodegrad-ability and capability to remove metals at mildly acidic condition,hence requiring less acid.The bioleaching process,although it seems to give a higher yield of metal extraction with lower chemical cost than chemical extraction,may be limited by the inability of the system to cope with the natural environmental conditions,requires strict monitoring of aeration rate and temperature and has applicability to only low sludge solids concentration.A full-scale study would be useful to better assess the e?ciency of the process.The electroreclamation technology is limited by its relatively higher energy consumption and limited applicability to sludge.The SFE method,on the other hand,is limited by the complexity of the process and the cost of ligands suitable for e?ective metal extraction.Both of these technologies are still in their early stage of application and hence there is a need for further basic and applied studies.Finally,the common advantage for almost all treatment technologies studied is that the extraction e?ciencies for some metals are high enough to remove metals from sludge to levels suitable for land application.ó2005Elsevier Ltd.All rights reserved.

1.Introduction

Wastewater treatment plants usually generate millions of tons of residual sludges worldwide every year.The man-agement of this sludge is a major part of waste treatment,

involving substantial cost and e?ort.For instance,esti-mates of 1999sludge production in the UK were close to 1.2million tds (Twigg et al.,2002)and were predicted to double to 2.18million tds by the year 2006(Wilson,1998).Sludge production in other countries is similarly excessive.In the United States,about 6.2million dry tons per year of municipal sewage is produced.Sludge produc-tion in the European Union (EU)in 1997was about 5.9million tds per year (Marmo,2001).It is predicted that this

0956-053X/$-see front matter ó2005Elsevier Ltd.All rights reserved.doi:10.1016/j.wasman.2005.09.017

*

Corresponding author.Tel.:+6629869009x2307;fax:+66298691123.

E-mail address:sandhya@siit.tu.ac.th (S.Babel).

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Waste Management 26(2006)

988–1004

?gure will rise to10million tons by the year2005(Lue-Hing et al.,1998).In Taiwan,the volume of sludge origi-nating from municipal and industrial wastewater treatment plants was estimated to be2.9·107and7.3·107m3/year, respectively(Lo and Chen,1990).In Thailand,sludge pro-duction from14small wastewater treatment plants and6 central wastewater treatment plants,including septic tank sludge,in the Bangkok Metropolitan area is expected to reach up to63,000tds/year in2010(Stoll,1995;Eckhardt and Khatiwada,1998).In Sao Paulo State,Brazil,it was predicted that there would be an estimated sludge produc-tion of25,000dry metric ton/year in2005.In Europe,the progressive implementation of the Urban Wastewater Treatment Directive91/271/EEC in Member States of the European Economic Community(EEC)is resulting in an increase in the quantities of sewage sludge requiring disposal.This increase is mainly due to the practical imple-mentation of the Directive and the slow but constant rise in the number of households connected to sewers including the increase in the level of treatment(Europa,2005).In the developing countries,so far,almost all of the sludge is disposed of in open?elds due to the shortage of appro-priate disposal facilities,resulting in serious problems and leaching of heavy metals to groundwater,surface water and soil(Marchioretto et al.,2002).

The basic disposal methods for such large quantities of sludge are land application,land?lling,incineration,ocean dumping and lagooning(Metcalf and Eddy,2003).Until a few years ago,sewage sludge could be re-used directly in agriculture as fertilizer.Recently,however,there has been an increased concern because of the legal criteria for the heavy metal concentration in sewage sludge(Lue-Hing et al.,1998;Scheltinga,1987).Concern for heavy metals is due to its non-biodegradability,toxicity and consequent persistence(Dutta,2002).Reduction of heavy metals in sewage sludge can be achieved either by source control of industrial and domestic discharges to the sewer systems or by extractive removal of metals from the sludge.Source control includes control of the processes and materials used in production at the industries;removal and controlled dis-posal of hazardous constituents before they reach the waste stream;separation of highly contaminated industrial wastewater from the domestic wastewater;and pretreat-ment of the wastes before discharge to the municipal collec-tion system.The major di?culty in source control arises in identifying the sources,which leaves metal removal from the sludge as the only practical solution(Sreekrishnan et al.,1993).It was reported that the total heavy metal con-tent of sewage sludges is about0.5–2.0%on a dry weight basis and in some cases may rise up to4%on a wet weight basis,especially for metals such as cadmium(Cd),chro-mium(Cr),copper(Cu),nickel(Ni),lead(Pb),and zinc (Zn)(Jain and Tyagi,1992).These heavy metals can come from industrial and domestic discharges,stream runo?and groundwater in?ltration from soil to the sewage system (Koch et al.,1982;Wozniak and Huang,1982).In the USA,for instance,sludges from50%to60%of the muni-cipal wastewater treatment plants cannot be applied on agricultural land because the Cd content of the sludge exceeds the standard(Couillard and Mercier,1990).Recent developments in waste management have also led to a decree which forbids the land?lling of solid wastes with organic matter content higher than5%(Veeken and Ham-elers,1999).Incineration,on the other hand,causes air pol-lution and hence requires expensive o?-gas treatment (White et al.,1995).Compared to land?lling and incinera-tion,utilization of sludge for agricultural use is the best alternative for sludge disposal because it recycles both nutrients and organic matter,once the heavy metals in sludge are reduced(Sreekrishnan et al.,1993;Veeken and Hamelers,1999;Levlin,2004;Pogrzeba et al.,2004).The extraction of heavy metals in sludge before land applica-tion is therefore a favorable step to achieve a more sustain-able form of sludge management.

In recent years,e?ective methods for heavy metal removal from sludge have been extensively studied(Wong and Henry,1988).Various technologies used for the extrac-tion of heavy metals in sludges have previously been used for the removal of metals from soils.In this paper,the application of chemical extraction,bioleaching process, electrokinetic process,and supercritical extraction in removing heavy metals from contaminated sludges is pre-sented.Moreover,a comparative review of the e?ciency of these various technologies,as well as critical analysis in terms of their advantages,disadvantages and limitations, is also done.

The measurement of the total metal content of sewage sludges and related matrices using analytical methods is well established.However,while total concentrations of metals indicate the extent of contamination,they provide less infor-mation into the forms in which the metals are present in sludge or their potential for mobility and bioavailability once released to the environment(Lester et al.,1983;Lake et al.,1984;Lake,1987).Hence,a detailed knowledge of the speciation of metals in the sludge itself and the changes in speciation likely to occur following disposal is also neces-sary to determine the suitability of decontaminated sludge for land application.This paper also discusses the chemical speciation of the metals present in sludges.

2.Chemical speciation of heavy metals in sewage sludge

The sequential chemical extraction(SCE)methods are widely used for the chemical speciation of heavy metals in sludge.It includes the utilization of a series of chemical extractants in a sequence of reagents of increasing strength. For each extraction step,a particular chemical form of the metal is expected to dissolve(Lake,1987;Marchioretto et al.,2002).The di?erent forms of metals which may occur in sludges as de?ned by several researchers have been vague with the de?nition of one often overlapping that of another.

Early studies on speciation were done by Stover et al. (1976),who used the SCE scheme incorporating potas-sium nitrate(KNO3),potassium?uoride(KF),sodium

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pyrophosphate(Na4P2O7)in anaerobically digested sew-age sludge from wastewater treatment plants in12cities in Indiana(USA).Oake et al.(1984)using the modi?ed scheme devised by Stover et al.(1976),also conducted speciation studies on selected types of sewage sludge(pri-mary,activated sludge and co-settled sludge before and after mesophilic anaerobic digestion)from major UK treatment https://www.wendangku.net/doc/6718135097.html,ing the same extraction procedure, Ali(1994)and Ratanachoo(1995)determined the di?er-ent forms of metals in anaerobically digested sludges from two treatment plants in Thailand.Marchioretto et al.(2002)also employed SCE to determine the distribution of heavy metals in the sludge matrix while assessing the chemical extraction process e?ciency in the removal of heavy metals from anaerobically digested sludge from a wastewater treatment plant in the Netherlands.The di?er-ent SCE methods developed by Tessier et al.(1979),Vee-ken(1998)and Sims and Kline(1991)were used to identify the forms of heavy metals present in the sludge. The results of these studies are summarized in Table1.

As shown,there is a wide variation in the forms of metals present in sewage sludges studied.Based on the selection of

Table1

Chemical speciation of heavy metals in anaerobically digested sludges

Metal Metal forms Reference

Cd Carbonates>sul?des>organic bound>

adsorbed=exchangeable

Stover et al.(1976)

Mainly carbonate Oake et al.(1984)

Residual(67.23%)>carbonate(32.77%)Ali(1994)

Mainly bound to Fe/Mn(Tessier);bound to organic

and inorganic matter(Veeken);organically bound and

present as inorganic precipitate(Sims and Kline)

Marchioretto et al.(2002)

Cr38–62%in organic fraction Oake et al.(1984) Residual(73.91%))organic(17.39%)>carbonate(8.7%)Ali(1994)

Organic(59%))residual(20%)>carbonate(16%))

adsorbed(4%)>exchangeable(1%)

Ratanachoo(1995) Mostly bound to organic matter(Tessier);bound to organic

and inorganic matter(Sims and Kline)

Marchioretto et al.(2002)

Cu Sul?des>carbonates>organic bound=adsorbed>exchangeable Stover et al.(1976) 43–70%in sul?de fraction Oake et al.(1984)

Residual(62.58%))carbonate(20.55%)>organic(13.5%))

adsorbed(3.07%)>exchangeable(0.31%)

Ali(1994) Organic(45%))residual(19%)>adsorbed(16%)>carbonate

(12%)>exchangeable(8%)

Ratanachoo(1995)

Mainly bound to organic matter(Tessier);essentially incorporated

in organic matter and organic-mineral aggregates(Veeken);mainly

bound to organic matter(Sims and Kline)

Marchioretto et al.(2002)

Pb Carbonates>organic bound>sul?des>adsorbed>exchangeable Stover et al.(1976) Predominantly in the organic and carbonate fraction Oake et al.(1984)

Carbonate(51.57%)>organic(33.96%))residual(11.32%))

exchangeable(2.52%)>adsorbed(0.63%)

Ali(1994) Organic(57%))carbonate(21%))adsorbed/exchangeable(9/8%)>residual(6%)Ratanachoo(1995)

Almost80%in residual portion(Tessier);mostly bound to organic

and inorganic matter(Veeken);present as inorganic precipitate(Sims and Kline)

Marchioretto et al.(2002)

Mn Organic(46.96%)>carbonate(38.44%))residual(12.90%))

exchangeable(1.46%)>adsorbed(0.24%)

Ali(1994) Organic(80%))carbonate(11%)>adsorbed/exchangeable(3/3%)>residual(2%)Ratanachoo(1995)

Ni Carbonates>organic bound>exchangeable>adsorbed>sul?des Stover et al.(1976) >40%in exchangeable/sorbed fractions Oake et al.(1984)

Residual(63.04%))carbonate(28.26%)>organic(8.70%)Ali(1994)

Residual(40%))organic(29%)>carbonate(15%)>exchangeable(10%)>adsorbed(7%)Ratanachoo(1995)

Especially bound to Fe/Mn(Tessier);bound to organic and inorganic

matter(Veeken);in the residual and bound to organic

matter(Sims and Kline)

Marchioretto et al.(2002)

Zn Organic bound>carbonates>sul?des>adsorbed>exchangeable Stover et al.(1976) Predominantly in organic fraction Oake et al.(1984)

Organic(65.84%))carbonate(22.06%)>residual(11.32%))

adsorbed(0.41%)>exchangeable(0.37%)

Ali(1994) Organic(58%))exchangeable(19%)>residual(9%)>carbonate/adsorbed(7/7%)Ratanachoo(1995)

Prevalent in the fraction bound to Fe/Mn oxides(Tessier);bound

to organic and inorganic matter(Veeken);present as inorganic precipitate(Sims and Kline)Marchioretto et al.(2002)

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work presented in Table1,the following forms of metals seem to predominate:Cd in carbonate and residual forms; Cr in organic and residual forms;Cu in residual and organic forms;Pb in carbonate and organic forms;Mn mostly in organically bound form;Ni in carbonate and residual forms;and Zn mostly in organically bound form.Oake et al.(1984)found that each individual metal had a charac-teristic fractionation pro?le independent of the type of sludge,with metals in the exchangeable,adsorbed and organically-bound fraction likely to be comparatively mobile once disposed to land.Moreover,mobilization of metals could result from dissolution of the carbonate frac-tions of Cd,Pb,and Ni or oxidation of the sul?de fraction of Cu.Furthermore,as organic matter decays to low levels, the nature of the major inorganic constituents in sludge(Fe, Al,Ca,or P),as well as the properties of the soil receiving the sludge,will ultimately a?ect the mobility of heavy met-als disposed to land(McBride,1995).Ali(1994)found that residual,organic and carbonate fractions contained more than95%of metals studied,whereas metals in exchangeable and adsorbed forms were negligible(less than5%).The results of the study done by Ratanachoo(1995),on the other hand,revealed that although most of the heavy metals studied were associated in residual form,the metals in adsorbed and exchangeable forms contained more than 10%of the total metals studied.Marchioretto et al.(2002) compared the heavy metals behavior in all of the fractions among the three SCE schemes used,and found that the dis-tribution in each fraction is almost the same for both Vee-ken,and Sims and Kline schemes,although it was di?cult to distinguish between what is bound to organic and what is bound to inorganic matter in the Veeken scheme.On the other hand,the Tessier and Veeken schemes were di?er-ent in several aspects.

In general,the forms of heavy metals present in sewage sludges vary widely according to the nature of the individ-ual metal,the characteristics of the wastewater treated and the sludge treatment employed(Lester et al.,1983;Lake, 1987).The disadvantages of the SCE scheme as cited by some researchers include its lack of speci?city;absence of selectivity;readsorption;and dependence on many factors such as type of sample,size of particulates,pH,tempera-ture,contact time,concentration of extractant,and solid–liquid ratio.These limitations have been con?rmed by Nirel and Morel(1990)and Marchioretto et al.(2002). 3.Metal extraction technologies

3.1.Chemical extraction process

Chemical extraction is a process of separating contami-nants from soils,sludges,and sediments by using extracting chemicals to reduce the volume of hazardous wastes for treatment.The process has been used in the remediation of soils contaminated with heavy metals(Pa?et al., 2004).The extractants can be inorganic acids,organic acids,chelating agents,and some inorganic chemicals.If acid is added to the sludge,the heavy metals present in the sludge can be dissolved and then exist in solution. The process is brought about by the exchange of the pro-tons(from the acid)by solubilization of heavy metals in sludge as shown in Eq.(1).Hence,one of the most impor-tant factors a?ecting the solubilization of metals in sludge is pH,as cited by Fristoe and Nelson(1983)and Waid-mann et al.(1984).Strong chelating agents such as EDTA (ethylenediaminetetraacetic acid)and NTA(nitrilotriacetic acid)function by complexing heavy metals to form EDTA-metal(or NTA-metal)complex(Eq.(2)).The reaction can be expressed as follows(Lo and Chen,1990):

Sludge–MtAcid!SludgetM zttAcide1TSludge–MtEDTA=NTA!Sludget

EDTA=NTA–Metal Complexe2TAfter extraction,heavy metals are removed from the extracting solution to recover the extracting agent and pre-vent environmental impact associated with the discharge of extracting?uid.Removal of the solubilized heavy metals from the extracting solution can be accomplished by pre-cipitation process followed by a separation https://www.wendangku.net/doc/6718135097.html,mon precipitating reagents include alkalis such as CaO,NaOH, NaHCO3,and sul?des such as NaS,H2S,or FeS(Brooks, 1991).The most appropriate and economically feasible technologies for removal of heavy metals from solutions are chemical sul?de precipitation and selective ion-ex-change(Veeken and Hamelers,1999).

3.1.1.Heavy metal extraction with inorganic and organic acids

Using inorganic acids,Jenkins and Scheybeler(1981) assessed the removal of metals from sludges obtained from a central wastewater treatment plant in California,USA. The types of sludge used in the experiment were primary sludge(PS)containing2.92%solids;digested primary sludge (DPS)containing 4.66%solids;waste activated sludge (WAS)containing0.68%solids;and digested waste activa-ted sludges(DWAS)containing1.54%solids.The sludges were dosed with sulfuric acid(1N H2SO4)to e?ect metal solubilization.The results revealed that the most di?cult metal to solubilize for any of the sludges was Cu(only about1%removal),followed by Cd and Pb.This suggested that part or all of the Cu in the sludges exists as an organic complex and that the organic-metal bond strength is greater than Cd and Pb(Wozniak and Huang,1982).In general,the experiment showed that if the pH is decreased to about2for contact time of24h,the maximum removal e?ciencies for Cd,Cr,Fe,Ni and Zn were found to be about70%or greater.Removal e?ciency for Pb was less at only13%.Preliminary estimates indicated that about 0.5metric ton of acid would be required for each dry metric ton of sludge solids for metal removal of more than50%of the Cd.Based on US$52.27/metric ton H2SO4,the total cost for acid treatment of PS and DPS at maximum metal removals was determined to be US$25.88and US$37.08/ dry metric ton,respectively.

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Using hydrochloric acid(HCl)as an extracting agent to remove metals from a waste activated sludge sample obtained from a treatment plant in Wisconsin,USA,Woz-niak and Huang(1982)determined the variables a?ecting the metal removal e?ciency.They observed variations in metal removal with pH,sludge solids concentration,indi-vidual metals,and time of extraction.They found that sol-ubilization increased with increasing time,up to approximately12h,and with lower values of pH and sludge solids.The rate of metal extraction was also found to be related to the possible metal species present.Up to 15%of the nickel and chromium extracted was solubilized almost immediately after acid addition,indicating dissolu-tion of inorganic precipitates.The initial removals of other metals such as Cd,Cu,Pb and Zn,which were less than 10%,indicated the presence of these metals in organome-tallic complexes.At approximately pH2,as much as 100%Zn removal was attained.The removal e?ciencies for other metals were92.5%for Cd,88%for Ni,73%for Cu,65%for Pb and only24%for Cr.

Logan and Feltz(1985)also used HCl to observe the e?ect of aeration,cadmium concentration and solids con-tent on acid extraction especially of Cd from an anaerobi-cally digested sludge obtained from a municipal wastewater treatment plant in Ohio,USA.The average solid content of the sludges used was3.3%.They found that extended aer-ation for at least14days was necessary for the oxidation of organic and inorganic forms of Cd prior to acid extraction, and that aeration during acid extraction was su?cient to maintain the metal in the extractable form.At pH2and leaching time of18h,an average of76%of Cd was removed.Other metals removed were Zn(77%),Mn (75%),Ni(70%),and Cu(26%).Not much removal was attained for Fe(15%),Pb(4%),Cr(2%),and Al(1%).

Lo and Chen(1990)explored the removal e?ciencies of heavy metals from urban and industrial sludges in Taiwan, using sulfuric acid(H2SO4)at4h contact time and twice the stoichiometric dose(one stoichiometric dose equals one equivalent acid per one equivalent of metals).The thickened aerobic digested sludge was taken from a muni-cipal treatment plant in Taipei City,while the anaerobic digested sludge was taken from an industrial district waste-water plant in north Taiwan.Results of acid extraction indicated that better removal e?ciencies were obtained at pH1.5or2.For the municipal sludge,Cd and Zn were the most readily solubilized metals with removal>95%. For the industrial sludge,the removal percentages of Cd, Cr and Ni were>97%.The remaining amount of most met-als in the decontaminated sludge was within the U.S.EPA 1993standards for sludges used in agriculture.

Blais et al.(1992a,b)also studied the removal of heavy metals from municipal sewage sludge(1.69–31.44g/L sol-ids)by acid treatment.The sludge samples were collected from11wastewater treatment plants in Canada and the USA.A total of23sludges of the following sludge types were used:secondary activated sludge;aerobically digested sludge;sludge from an oxidation pond;anaerobically digested primary sludge;anaerobically digested secondary sludge;primary sludge;and anaerobically digested sludge. The sludges were dosed with H2SO4at pH of1.5and a temperature of21°C for24h.It was found that Mn gave the highest yield of metal solubilization of83%,followed by Ni(68%),Zn(66%)and Cd(59%).It was also observed that the sludge types and solids contents used in the study did not considerably in?uence the leaching capacity of the acid extraction.

Using both organic and inorganic acids,Veeken and Hamelers(1999)conducted an acid leaching study on the removal of heavy metals from anaerobically digested sew-age sludge with20%dry matter(DM)collected from a municipal sewage treatment plant in the Netherlands.The metal extraction was performed at pH range of2–6for 0.1M oxalic acid;0.1M citric acid;and nitric acid (HNO3)at room temperature.Only two heavy metals(Cu and Zn)and competing metals(Ca and Fe)were measured during extraction.The results revealed that both citric and oxalic acid had increased heavy metal extraction(70%and 60%,respectively,for Cu;90%and70%,respectively,for Zn)at a mildly acidic pH of3–4as compared to HNO3 (38%and65%,respectively,for Cu and Zn)at pH of1.5–2.The extraction e?ciencies for the two metals were high enough to remove metals from sewage sludge originating from agro-industries to levels below the Dutch legal stan-dards of75mg/kg DM and300mg/kg DM,respectively. Citric acid,being tri-carboxylic,was found to be better than oxalic acid,which is di-carboxylic,because oxalic acid is removed from solution by precipitation as calcium oxalate. Moreover,oxalic acid is a strong reducing agent that might become oxidized in an organic matter matrix.This causes the decrease of oxalate ions available for leaching,resulting to a lower extraction for the metals compared to citric acid.

Marchioretto et al.(2002)assessed the chemical extrac-tion e?ciency of both organic and inorganic acids in the removal of heavy metals from anaerobically digested sludge (25g/L DM)from a wastewater treatment plant in the Netherlands.The plant received both industrial and muni-cipal wastewater as in?uent.Nitric,hydrochloric,oxalic and citric acids were used in the experiment.The results indicated that oxalic acid did not show good results com-pared to the other acids.It is less soluble than citric acid at the same pH value.At higher pH values(3and4),citric acid achieved the highest extraction e?ciency of85%for Cd,Cr and Zn.This was probably due to its chelating prop-erties.It was also observed that at pH values lower than3, the extraction e?ciency of HNO3or hydrochloric acid (HCl)is improved,achieving100%removal for Cd and Pb.Except for Cu(for organic acids)and Cr,Cu and Zn (for inorganic acids),the heavy metals in decontaminated sludge were well within the Dutch legal standards.

3.1.2.Heavy metal extraction with chelating agents

Jenkins and Scheybeler(1981),using the four slud-ges mentioned in Section3.1.1from a central wastewater treatment plant in California,studied the extraction of

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heavy metals also using EDTA,at1h contact time and twice the stoichiometric dose.Based on metals solubiliza-tion,results revealed the maximum removal e?ciency of 37%for Pb,and8%for Cu.The corresponding maximum removal e?ciencies for Fe,Zn,Ni and Cr were89%,45%, 53%,and33%,respectively.The solubilized metals were recovered by precipitation with lime,after pH adjustment to7.0with Ca(OH)2.

Lo and Chen(1990)used EDTA and NTA to solubilize heavy metals from municipal and industrial sludges in Tai-wan after4h contact time and twice the stoichiometric dose (2mol of EDTA/NTA per mole of metal).EDTA extraction results showed that the removal e?ciency was dependent on the nature of the sludge.For municipal sludges,the order of solubilization was Pb>Cd>Zn>Ni%Cu>Cr.For the industrial sludges,the order was Cd>Ni>Pb>Cr> Zn>https://www.wendangku.net/doc/6718135097.html,pared with acid extraction,the EDTA removal for Pb was higher,while removal for Cr,Ni and Zn was lower.Cd and Cu removals were,however,nearly equal.For NTA extraction,the order of metal solubilization from the municipal and industrial sludges was Cd% Ni>Pb>Zn%Cu%Cr.These di?erences in the resulting order of solubilization re?ected the e?ects of the nature or forms of metals in the sludges as discussed earlier in Section 2.The remaining amount of heavy metals in decontami-nated sludges was well within the US EPA standards.

3.1.3.Heavy metal extraction with inorganic chemicals

Zhiping(1995)investigated the e?ect of leaching by ferric chloride(FeCl3)of heavy metals from anaerobically digested sludge(11.29%total solids)and undigested sludge cake(20.82%total solids)that were collected from a sewage treatment facility in Bangkok,https://www.wendangku.net/doc/6718135097.html,ing FeCl3?6H2O,leaching was done using a laboratory shaker at solu-tion pH of3for4–7days.Results revealed Zn solubilization was the largest at92%and65%,respectively,for the anaer-obically digested sludge and undigested sludge cake.The decontaminated anaerobically digested sludge and undi-gested sludge contained about412.4and881mg/kg dry sol-ids(DS),respectively,well below the proposed Thai standard of3000mg/kg dry solids for Zn.Removals attained for other metals were30%for Cu,59%for Mn and39%for Ni,for anaerobically digested sludge,while for undigested sludge cake metal removals were46%for Cu,51%for Mn and51%for Ni.In general,the experiment revealed that for anaerobically digested sludge,lower pH, lower solid content and higher ferric ion dosage favored metal solubilization,although the strong bu?er capacity of the sludge seems to make FeCl3less e?ective.For undi-gested sludge,it was assumed that its weak bu?er capacity resulted in a drastic drop of sludge pH when ferric salt was added,favoring the dissolution of acid soluble heavy metals and oxidation of oxidizable heavy metals.

3.1.

https://www.wendangku.net/doc/6718135097.html,parison of extracting agents

Table2shows a summary of the e?ciencies of the dif-ferent extracting agents used to solubilize heavy metals from sludge.Initial metal concentration in mg/kg of dry solids,is provided wherever available in order to show the wide variation in the metal content of various types of sludges.

As shown,for inorganic acids,as much as100%removal was attained for Cd,Pb and Zn using HCl and H2SO4as extracting agents.For Cr,the maximum removal of97% was attained using H2SO4at pH 1.5while maximum removal of99%was attained for Ni,using the same reagent.Cu,on the other hand,attained only as much as 80%removal using HCl and HNO3.

The use of chelating agents resulted to lesser removal e?ciencies for most of the metals except for Cd and Ni which attained as much as98%and95%,respectively, using NTA.

For organic acids,the removal e?ciency attained was relatively good at higher pH conditions compared to inor-ganic acids and chelating agents.In comparison with oxalic acid,citric acid attained higher removal e?ciency for all metals studied.

Compared to other extracting agents,the use of inor-ganic chemicals such as ferric chloride,resulted to lower removal e?ciencies for most metals,except for Zn,which is comparable to that attained using organic acids.

In summary,it seems that inorganic acids are superior over the other extracting agents in removing heavy metals from sludge at a lower pH of1.5–2.However,like chelating agents and other inorganic chemicals,the use of inorganic acids can produce solid and liquid waste streams which can have negative impacts in the https://www.wendangku.net/doc/6718135097.html,anic acids can be more promising because the extraction can be per-formed at mildly acidic conditions(pH3–4).Moreover, organic acids are biodegradable which implies that the decontaminated sludge does not have to be conditioned, resulting in a substantial reduction of wastewater required for washing the sludge.

3.2.Bioleaching process

Bioleaching was originally used as a method where microorganisms are applied to extract metals from ores. With bioleaching,solubilization of metals occurs by the metabolic action of microorganisms(Ratanachoo,1995). The bacteria which are involved in the extraction of metal-lic compounds from the mineral source are called chemo-lithotrophic microorganisms.Thiobacillus ferrooxidans is the most well-known species among these microorganisms (Dispirito et al.,1982;Solisio et al.,2002).T.ferrooxidans has an optimum pH range of1.5–4and grows best at a pH of about2.The ability to resist low pH is an important physiological characteristic since sulfuric acid is generated when reduced inorganic sulfur compounds are being oxi-dized(Blais et al.,1992a,b).The main reactions taking place in the process can be summarized into two processes, namely:indirect and direct modes.The indirect process involves the following reactions(Torma and Banhegyi, 1984;Hutchins et al.,1986):

S.Babel,D.del Mundo Dacera/Waste Management26(2006)988–1004993

2FeSO 4t0:5O 2tH 2SO 4 !T .ferrooxidans

Fe 2eSO 4T3tH 2O e3T

4Fe 2eSO 4T3t2MS t4H 2O t2O 2 !2M 2tt2SO 2à4t8FeSO 4t4H 2SO 4e4T

where MS represents the metal sul?de and M 2+represents the soluble metallic ion.

Reaction (3)takes place under the active participation of Thiobacillus ,whereas reaction (4)takes place only chem-ically without any participation of bacteria.A cyclic pro-cess between reactions (3)and (4)results in more and more metal solubilization.The formation of H 2SO 4during the process further enhances the overall e?ciency.In the direct process,non-ferrous sul?des can be directly oxidized by T.ferrooxidans to soluble metals sulfates according to Eq.(5)below (Wong and Henry,1983):

MS t2O 2 !T .ferrooxidans T .thiooxidans M 2ttSO 2à

4

e5TBecause the metal sul?de exists in insoluble form and the metal sulfate (MSO 4)is usually water soluble,this transfor-mation enables the metal to move from solid phase to liquid phase in which further treatment is provided to recover the metal.Karaivko (1985)found that there is a direct relationship that exists between the rate of metal extraction and the solubility product of sul?de minerals.The higher the solubility product of its sul?de mineral,the faster is the rate of metal extraction.

The other microorganism involved in microbial leaching is Thiobacillus thiooxidans which is morphologically and physiologically similar to T.ferrooxidans .The fundamental di?erence between the two species is generally recognized by the inability of T.thiooxidans to oxidize ferrous iron and insoluble metal sul?des (Torma,1986).

Studies on the application of bioleaching in metal removal from sludges have been conducted in batch and continuous processes.

3.2.1.Batch bioleaching studies

Wong and Henry (1984)conducted batch bioleaching studies on an anaerobically digested sludge obtained from a wastewater treatment plant in Toronto,Canada,in which they tested the e?ect of aeration rate,pH and tem-perature on the metal removal e?ciency.The results revealed that the optimum pH of 4for metal solubilization corresponded to the optimum pH of T.ferrooxidans .At pH below 1.5and above 4,the rate of leaching was sup-pressed.Moreover,adequate solubilization of metals occurred during aeration at a rate of 50cm 3of air/L of sludge/min.T.ferrooxidans are strictly aerobic,and e?ec-tive bacteria leaching occurred only when there was a plen-tiful supply of oxygen.At 100cm 3of air/L of sludge/min,no major improvement in metal removal was observed.Also,the optimum temperature varied from 25to 35°C.Above 40°C,the rate of metal solubilization dropped sharply,and at 50°C it ceased.It was also found that the rate of metal solubilization decreased at low tempera-tures while the metal removal e?ciency was found to

increase by about 30%when the temperature was raised from 10to 25°C.These results con?rmed the ?ndings of Blais et al.(1993).At an initial pH of 4,an aeration rate of 100cm 3of air/L of sludge/min,a temperature of 25°C and leaching period of 8days,the range of metal concentration in the treated sludge were:25–35mg/kg DS for Cd;325–694mg/kg DS for Cu;125–158mg/kg DS for Ni,1275–1450mg/kg DS for Zn and 530–540mg/kg DS for Pb.These corresponded to the following removal e?ciencies:80–85%for Cd;66–80%for Cu;70–78%for Ni;84–90%for Zn;and only 0–9%for Pb.The resulting decontaminated sludge complied with the Ontario regulations for agricultural land disposal of sludge.Moreover,it was concluded that the factors a?ect-ing the bacterial leaching process in removing heavy metals from sewage sludge were aeration rate,pH,temperature,mixing,substrate concentration and the composition of the leaching medium.

Investigation of bacterial leaching of heavy metals from anaerobically digested sludge was also done in a batch sys-tem by Tyagi and Couillard (1987)and Tyagi et al.(1988)using a mixed culture of T.ferrooxidans and T.thiooxidans and a single culture of T.ferrooxidans alone.A mixed cul-ture process gave 10%higher metal solubilization com-pared to a single culture.In mixed culture,the maximum metal solubilization obtained for Zn,Cu,Pb,and Cd was on the order of 96%,75%,55%and 50%,respectively,in 10–12days.

Blais et al.(1992a,b)also studied the removal of heavy metals from municipal sewage sludge using microbial leaching and acid treatment in laboratory reactors.The sample sludge having a total solids content of 1.69–31.44g/L,was collected from 11wastewater treatment plants in Canada and the USA.In the bioleaching exper-iment,a total of 23sludges of the following sludge types were used:secondary activated sludge;aerobically digested sludge;sludge from an oxidation pond;primary sludge digested anaerobically;secondary sludge digested anaerobically;primary sludge;and anaerobically digested sludge.These sludges were leached with adapted sulfur-oxidizing micro?ora and elemental sulfur,as substrate.Microbial leaching with adapted iron-oxidizing micro?ora and ferrous sulfate as substrate and incubation tempera-ture of 21°C for 240h was also done.Eight metals were tested:Al,Cd,Cr,Cu,Mn,Ni,Pb,and Zn.The results revealed that Mn attained the highest yield of metal solubilization at 92%and 89%,respectively,for both the sulfur-oxidizing bacteria and iron-oxidizing bacteria;followed by Zn at 85%and 82%,respectively.Cd attained 82%and 74%,respectively,for both sulfur-oxi-dizing bacteria and iron-oxidizing bacteria.The mean sol-ubilization obtained for the eight metals tested was 62.5%for bioleaching with sulfur as substrate,and 49.5%with ferrous sulfate as substrate.It was concluded that the indigenous sulfur-oxidizing bacteria or iron-oxidizing bac-teria can be utilized for toxic metals removal from sewage sludge.

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995

Hsin-Jung(1994)investigated the e?ects of pH,solids concentration and ferrous iron concentrations on heavy metals solubilization rate and the overall removal e?ciency from bioleaching study done on anaerobically digested sewage sludge containing2.0%solids.The sample sludge was collected from a central wastewater treatment plant in Bangkok,Thailand,which receives a combined sewage from commercial and domestic sources.The microorgan-ism used in the process was T.ferrooxidans using FeS-O4?7H2O as the energy source,cultivated at30°C and pH2.5.Results of the study revealed the following heavy metal solubilization:Cu,80.71%;Zn,91.61%,Mn, 92.44%;Ni,95.0%;Cr,33.09%;and Cd,41.67%.It was concluded that ferrous iron,which served as the energy source of bacterial growth,in?uences the metal solubiliza-tion rate and metal removal e?ciency.High ferrous iron concentration leads to high heavy metal leaching rate and high removal e?ciency.Moreover,it was demonstrated that bacterial leaching was e?ective in removing Cu,Zn, Mn,Ni,Cr,and Cd from anaerobically digested sludge at pH lower than4.5.Furthermore,solids content also a?ected the heavy metal solubilization rate.Generally, increase of solid concentration will decrease the metal removal e?ciency because high solids content leads to a reduction of the mass transfer of O2or CO2required by T.ferrooxidans for biosynthesis.Also,in a batch system, the accumulation of high heavy metal concentration lea-ched from high solids content sludge may reach levels that can become toxic to microorganisms.

Other researchers commented that studies focused on trials in batch reactors were non-economically viable since more time(6–12days)is required for most studies to obtain80%of Cu,Zn,Cd and Ni solubilization(Tyagi and Couillard,1987,1989;Couillard and Mercier,1991), hence more bioleaching studies were being done using con-tinuous reactors.

3.2.2.Continuous bioleaching studies

Couillard and Mercier(1990)studied the microbial leaching of heavy metals from anaerobically digested sludge,using a single culture of T.ferrooxidans and ferrous sulfate(FeSO4?7H2O)as substrate in a continuously stir-red tank reactor,without recycling(CSTR)and with sludge recycling(CSTRWR).The sludge,which contained2.9–3.2%total solids(TS),was collected from a central waste-water treatment plant in Canada.The study was conducted at residence times of1–4days.Results of the study revealed that at a lesser amount of substrate(e.g.,1g/L),CSTRWR is supposed to be more e?cient than CSTR,but because of copper reprecipitation or recomplexation in the settling tank,its e?ciency just equaled that of CSTR.However, when3g/L FeSO4?7H2O was added,about62%of Cu and about77%of Zn were dissolved in CSTR in3days res-idence time,compared to50%of Cu and64%of Zn in the CSTRWR.It was concluded that a larger amount of sub-strate makes CSTR more e?cient than CSTRWR.More-over,hydraulic retention time(HRT)and pH were found to be the main factors for Zn solubilization,while for Cu,the redox potential(ORP)was also a major factor. Furthermore,increased concentration of ferrous substrate yielded better Cu solubilization while it had no e?ect on Zn.

Tyagi et al.(1990)also observed the e?ect of HRT and recirculation ratio in the bioleaching https://www.wendangku.net/doc/6718135097.html,ing a sin-gle culture of T.ferrooxidans,metal solubilizations achieved at0.75day HRT with20%recirculation were: 91%Cu,94%Zn,93%Mn,67%Cd,67%Ni,8%Cr and 7%Pb.

Ratanachoo(1995)conducted a laboratory bioleaching study in a CSTR using a pure culture of T.ferrooxidans to leach anaerobically digested sludge cake(1.5–2%TS) from a treatment plant in Bangkok,Thailand.With the optimum operational conditions of1day HRT,20%sludge recirculation and4g/L ferrous sulfate concentration,the study resulted in the following metal solubilization:6.7% Cr,73.78%Cu,66.94%Mn,52.72%Ni,2.89%Pb and 86.49%Zn.The study concluded that the bioleaching pro-cess is a?ected by HRT,sludge recirculation and ferrous sulfate concentration.Longer HRT resulted in slightly higher solubilization rate.For instance,the percentage of solubilization of Cu at3days HRT,was72.91%,while at 0.5day HRT,only63.78%.Sludge recirculation of20% slightly increased solubilization of Cr,Cu and Mn at0.5 and1day HRT;Ni and Pb only at1day HRT,while no e?ect was observed for Zn at both HRTs.The concentra-tion of ferrous sulfate also signi?cantly a?ected the solubi-lization rate of Cu,Mn,Ni and Zn,whereas Cr and Pb were slightly a?ected.An increase in ferrous sulfate concen-tration resulted in an increase of solubilization rate while decreasing the pH.

The results of the aforementioned studies done on biole-aching process are summarized in Table3.

3.3.Electroreclamation studies

The electroreclamation technique has been developed and applied in some?eld trials and cleanup projects during the past12years to remove toxic metals from contami-nated soils(Acar and Alshawabkeh,1993;Zagury et al., 1999).It has also been applied for the removal of heavy metals and organic pollutants from sludges and sediments (Jeong and Kang,1997).The method is based on electroki-netic phenomena that occur when the soil/sludge is electri-cally charged with direct current by means of one or several electrode arrays:electrophoresis,electroosmosis,and elec-trolysis(Acar and Alshawabkeh,1993;Alshawabkeh and Bricka,2000).The transport of heavy metals can be caused by all these phenomena,the most important being electrol-ysis or electrically induced ion migration.The extraction and removal of metals can be accomplished by electrode-position,precipitation,or ion exchange either at the elec-trodes or in an external extraction system.

Mohamed and Saleh(1997)used electrokinetics to investigate the mobility and removal of heavy metals from

996S.Babel,D.del Mundo Dacera/Waste Management26(2006)988–1004

fresh and older dewatered municipal wastewater sludge (30%TS).The sample sludge was collected from a waste-water treatment plant in Montreal,Canada,which treats both municipal and pretreated industrial wastewater.The experiment was carried out in a plexiglass cylindrical cell in which a constant voltage of26.5V was applied.Results of the study revealed that the application of electric poten-tial can successfully remove heavy metals from dewatered municipal wastewater sludge.The process is most e?cient when applied to fresh sludge where excessive gas genera-tion lowers the pore?uid pH within the sludge and increases heavy metals solubility and mobility.Percentage removals achieved for Ni,Cr,Zn and Cu were41%,8%, 6.7%,and4.7%,respectively.Moreover,for the anodic e?uent,Cr showed an exceptional tendency of higher removal from the anodic site under the e?ect of low pH.

A similar initial tendency for Cu was observed but was soon in?uenced by the osmotic?ow.However,for older sludge,lower metal removals were achieved even with the introduction of HNO3,at pH4.0.Results showed further that,the distribution of heavy metals within the sludge showed high Ni and Zn mobilities toward the cathode as re?ected in its high concentration gradient across the cell. Cr and Cu concentrations in the middle layer were the highest,which can be attributed to lower rates of migration toward the cathode thus accumulating in the middle layer. In general,heavy metals redistribution within the sludge re?ected high mobility of Ni followed by Zn,Cr,and Cu.

Zagury et al.(1999)conducted an ex situ pilot-scale study of electroreclamation of incinerator sludge.The sam-ple sludge was collected from a gas plant in France.The sludge,which was mostly silt and clay(90.6%),had a solid content of52%,carbonate content of5.7%and pH of 10.35.Prior to the electroreclamation study,sequential chemical extraction was performed to determine the di?er-ent forms of metals in the sludge.SCE results revealed that the heavy metals contained in the sludge were mainly found in residual fraction(80%).Metals in this fraction are con-sidered to be bound within the lattice of the minerals and can become solubilized and available for electromigration only after digestion with strong acids at elevated tempera-tures.The electrokinetic experiment was carried out in a pilot scale batch cell.A direct current(DC)power source was used to provide a constant2.5ampere(A)current strength,resulting in a current density of3mA/cm2.Pre-acidi?cation was done with nitric acid(HNO3)to decrease the pH of the sludge to 4.5.Pre-acidi?cation induced desorption of metals with the following amount of metals extracted:Zn(5.6%);Cr(13.7%);Ni(14.1%);Cu (12.2%);Mn(25.8%)and Fe(23.4%).Overall extraction results achieved were38.6%for Zn,51.2%for Cu and 71.8%for Mn.The removal e?ciency for the other metals could not be exactly determined since extra metals might have been introduced by the materials used in the experi-ment resulting in an unusually high concentration of metals in the cathode.With the current intensity of2.5A,an aver-age voltage of14.8V,and a treatment duration of47days, the calculated energy used in the experiment was about 41.7kW h(1.5·108J)equivalent to about1.241kW h/m3 for humid sludge with a total cost(including chemicals) of about US$100/ton.The energy consumption is within the range of the energy levels for in situ pilot scale?eld studies reported by Acar et al.(1995)which is between 60and200kW h/m3.Based on the results of the study,it was concluded that chemical pre-acidi?cation improves sludge initial pH and electrical conductivity,therefore,pro-moting migration of metals.Moreover,the use of a high current permits a strong acidi?cation of the sludge which reaches very low pH values.Finally,the potential of elect-roreclamation to treat heavy metals contaminated sludge was con?rmed.

A summary of the results of the electroreclamation stud-ies is presented in Table4.

3.4.Supercritical?uid extraction

Supercritical?uid extraction(SFE)is essentially a liquid extraction process employing compressed?uids which are usually either liquids or gases,under supercritical condi-tions instead of normal solvents(Bishop,2000).

Extraction of metal species from solid and liquid mate-rials using SFE has been the subject of several recent stud-ies(Lin et al.,1995;Wang et al.,1995;Kersch,2004). Interest in supercritical?uids,particularly carbon dioxide,

Table4

Heavy metal removal data for electroreclamation studies

Operating conditions Maximum metal removal e?ciency(%)Reference

Cr Cu Mn Ni Pb Zn

In?uent pH4.08(6895)a5(1206)–41(1497)–7(677)Mohamed and Saleh(1997) Power source:26.5V

Sludge type:dewatered municipal wastewater sludge

Total solids:30%

Treatment duration:up to50days

In?uent pH4.5–51(230)72(7050)––39(3450)Zagury et al.(1999) Power source:14.8V

Sludge type:incinerator sludge

Total solids:52%

Treatment duration:47days

a()–Initial metal concentration in mg/kg dry solids.

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as a solvent for use in extraction processes has been driven by increased environmental legislation restricting the use of conventional solvents(Smart et al.,1997).Recently,super-critical?uids modi?ed by the addition of complexing agents have been utilized in the extraction of metal ions from various solid and liquid matrices(Wang and Mar-shall,1994;Wang et al.,1995).Initially,metal extraction using supercritical?uids had been considered unfeasible due to the charge neutralization https://www.wendangku.net/doc/6718135097.html,intz et al.(1991)observed that by addition of a complexing agent to the supercritical?uid,the charge on the metal ions can be neutralized and lipophillic groups are introduced to the metal complex system.Solubilization of the metal com-plex into the supercritical?uid is then possible.

Smart et al.(1997)investigated the use of a range of commercially available complexing reagents for the extrac-tion of heavy metals from a variety of matrices by super-critical?uids.They calculated that alkyl-substituted ligands with a chain length of approximately eight carbon units,preferably branched,will show the most favorable properties in terms of achieving high solubility in supercrit-ical CO2.They have,therefore,selected commercially available ligands with alkyl chain lengths of between eight and ten carbon units to extract a range of heavy metals from cellulose,sand,clay,wood,and soils.Five commer-cially available ligands were used in the experiment.Of these,the following sulfur-containing organophosphorus reagents were found to be e?ective for the extraction of heavy metals from a range of matrices:Cyanex301 [bis(2,4-4-trimethylpentyl)-dithiophosphinic acid],Cyanex 302[bis(2,4,4-trimethylpentyl)-phosphinic acid]and D2EHTPA[bis(2-ethyl-hexyl)monothiophosphoric acid]. Operating pressure and temperature of200atm and 60°C,respectively,were found to be the e?ective condi-tions to achieve extraction.Other ligands used were Kelex 100[7-(1-vinyl-3,3,5,5-tetramethylhexyl)-8-hydroxyquino-line]and Cyanex272[bis(2,4,4-trimethylpentyl)-phosphinic acid].The results revealed that without the addition of ligands,the metal ions cannot be extracted by pure supercritical CO2.The Kelex100ligand showed very high selectivity for the Cu2+ion,extracting100%,while the only other metal extracted in a signi?cant amount was Zn.The Cyanex301and D2EHTPA both showed the capability to extract all of the metals studied in quantitative amounts. The Cyanex302showed high extraction e?ciencies for Cu2+and Cd2+but intermediate with respect to Pb2+ and Zn2+.Cyanex272showed intermediate e?ciencies with respect to all of the metals studied.The Cyanex272 ligand is mainly used in supercritical extraction for the recovery and separation of cobalt and nickel(Tait,1993). For the overall result of the experiment,it was observed that the solubilities of the metal complexes were found to be in the range of0.15–12g/L,depending upon the ligand and the conditions studied.The results for the extraction of some metals from an inert matrix(cellulose)are shown in Table5.

Other ligands,such as crown ethers,were also used in SFE studies for heavy metal extraction.Wang et al. (1995)performed selective extraction of Hg2+from solid and liquid samples by supercritical CO2(5%methanol modi?ed)containing bistriazolo-crown ethers as extract-ants.The macrocyclic compounds(crowns1,2and3)were prepared by the reaction of3,5-bis(chloromethyl)-1-(tetra-hydro-2-pyranyl)-1H-1,2,4-triazole with appropriate cate-chols and trans-1,2-cyclohexanediol,respectively,in tetrahydrofuran(THF)and NaH as a base followed by an acid hydrolysis.These compounds are18-membered macrocytes with6donor atoms,an appropriate size suit-able for interaction with divalent heavy metal ions.The melting points of crowns1,2and3are258–260,240–242,and178–180°C,https://www.wendangku.net/doc/6718135097.html,ing cellulose-based ?lter paper spiked with mercury as a sample,as much as 98%extraction e?ciency was achieved with methanol-modi?ed CO2containing bistriazolo-crown3and with water present in the solid matrix.

Kersch(2004)is currently conducting a SFE experiment on sludges which involves the development of a large-scale set-up(with reactor volume of12L)to separate dissolved heavy metal complexes from CO2by a parallel ligand regeneration process.

Table5

Extraction e?ciencies with SFE of some metals from cellulose using supercritical CO2and dissolved ligands a(without nitric acid solutions)

Ligand Extraction e?ciency

Cu2+Pb2+Zn2+Cd2+

%Ext.b%Coll c%Ext.%Coll.%Ext.%Coll.%Ext.%Coll. None22203210 Kelex100100–d0–d10–d1–d Cyanex301828110085100899579 Cyanex3029893605632508577 Cyanex2725041521181773022

D2EHTPA981001008997769990 Source:Smart et al.(1997).

a A350mg portion of ligand was used in each case.The results represent the mean value obtained from duplicate runs under each condition studied.

b Percente%Textraction?metal loadedel gTàmetal remainingel gT

metal loadedel gT?100.

c Percente%Tcollection?metal collectedel gT

metal loadedel gT?100.

d No analysis was performed on thes

e extracted samples.

S.Babel,D.del Mundo Dacera/Waste Management26(2006)988–1004999

https://www.wendangku.net/doc/6718135097.html,parison of heavy metals removal technologies

In general,the treatment technologies employed for metal removal from contaminated sludge do not destroy the metals but only accomplish metal separation,thereby reducing the volume of contaminated sludge to be treated (Tyagi et al.,1991).Hence,the decontaminated sludge can be safely applied to agricultural lands for resource uti-lization,while the chemical sludge containing metals can be either disposed of in land?lls or further processed by chem-ical sul?de precipitation or ion exchange to recover and recycle the metals.

A study of the various technologies for the extraction of heavy metals from sludge showed a broad range in metal extraction e?ciencies.Wong and Henry(1988)and Lo and Chen(1990)attributed this to the di?erences in sludge composition,pretreatment of sludge,concentration and forms of metals in sludge,and extraction conditions.

In the treatment of contaminated sludge by chemical extraction,several extracting agents have been evaluated. These include inorganic acids such as H2SO4,HCl and HNO3;organic acids such as citric and oxalic acids;chelat-ing agents such as EDTA and NTA;and inorganic chemi-cal such as ferric chloride,each attaining di?erent extraction e?ciencies for the metals.A comparison of the treatment e?ciencies of these extracting agents has been discussed earlier in Section3.1.4.

Despite the high extraction e?ciencies achieved in the chemical extraction of most metals from contaminated sludge,most often,Cu was not solubilized.Other factors such as high acid requirements(0.5–0.8g acid per g of dry sludge)and operational di?culties such as the requirement of acid-corrosion resistant apparatus and need to regener-ate acceptable pH,also put constraints on the utilization of the chemical method compared to other extraction meth-ods such as bioleaching(Jenkins and Scheybeler,1981; Wong and Henry,1984;Tyagi and Couillard,1987,1989; Tyagi et al.,1988;Jain and Tyagi,1992).

Tyagi et al.(1988),in comparing the performance of chemical treatment with that of bioleaching in extracting metals from sludge,concluded that the bioleaching process gave a higher yield of metal extraction and that the chem-ical cost was found to be80%lower than in the chemical process because of lower acid and lime requirements. Moreover,in the microbial process,Cu can be decreased to an acceptable level at a comparatively higher pH of 3–4.They further concluded that the bioleaching process does not a?ect the soil conditioning and fertilizing values of the sludges.The volatile solids,nitrogen,phosphorus and potassium concentrations were found to be unchanged after bacterial leaching.This con?rmed the?ndings of Wong and Henry(1984).

Blais et al.(1992a,b),who compared the leaching poten-tial of sulfuric acid treatment and two microbial processes(a microbial leaching process with elemental sulfur and/or fer-rous sulfate as substrate),concluded that the use of micro-bial leaching process reduced considerably the quantity of acid required for metal extraction.This con?rms the conclu-sion of Tyagi et al.(1988)and Wong and Henry(1984).It was further concluded that for solubilization of all metals tested,the bioleaching process with sulfur as substrate was better than the acid treatment and the microbial leaching with ferrous sulfate.For solubilization of speci?c metals, the microbial leaching with ferrous sulfate as substrate per-mitted a better solubilization of Cd,Cu,Mn,and Zn than the sulfuric acid treatments.However,solubilization of Al, Cr,Ni,and Pb was less than that of acid leaching.For sol-ubilization of Cu and Ni,the bioleaching with sulfur-oxidiz-ing bacteria was better than with ferrous sulfate.

Although bioleaching seemed to o?er more advantages over acid treatment,this technology has its own limita-tions.So far,bioleaching studies have been done in the lab-oratory where reproductive rates of the bacteria can be increased by manipulating nutrients and physical condi-tions,including freedom from interference by antagonistic competitors.However,in natural environments,conditions are rarely conducive to optimal growth of selected microbes(Hsin-Jung,1994);hence,di?culty might be encountered in setting up a full-scale bioleaching study. Another limitation is that since T.ferrooxidans is strictly aerobic,e?ective bacteria leaching can only occur when there is a plentiful supply of oxygen.Bacterial leaching is also temperature dependent.Hence,failure to monitor these parameters might adversely a?ect the e?ciency of the bioleaching process.In addition,the bioleaching pro-cess seemed to be applicable only for low solids concentra-tion.From previous studies done,it was observed that an increase of solid concentration decreased the metal removal e?ciency of the bioleaching process.Besides,the accumu-lation of heavy metals resulting from increased heavy metal concentration leached from high solid contents sludge in batch system,may reach levels that can become toxic to microorganisms.

The e?ciencies for the removal of the metals by the dif-ferent extraction technologies reviewed are presented in Fig.1.As shown,Cd seems to be e?ectively removed by acid treatment as compared to the bioleaching process, although the SFE process also gives high removal e?ciency depending on the type of ligand used.Cr,Pb and Ni seemed to be also e?ectively removed by the acid treat-ment.For the removal of Cu,Mn and Zn,the bioleaching process seemed to be appropriate,which con?rms the results of Kaztanek et al.(1994),speci?cally for Zn.The SFE process also gives good results for Cu,Mn and Zn removal.The electrokinetic process exhibited better removal e?ciency for Mn,but is still inferior over acid treatment and bioleaching processes.

The newly emerging electrokinetic technology for extract-ing metals from sludge also gave good removal e?ciencies for some metals.However,previous laboratory studies done were mostly limited to experiments conducted on commer-cially available clay minerals,such as kaolinite and montmo-rillonite,by arti?cially introducing various contaminants (Hamed et al.,1991;Gale et al.,1994).The studies done

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on sludge have focused on dewatered sludge with higher sol-ids content.Another factor which might make this technol-ogy unattractive is the higher energy consumption of the system as compared to the other metal extraction technolo-gies.Zagury et al.(1999)reported an energy consumption of about1.241kW h/m3for humid sludge.

The supercritical?uid extraction(SFE)method,seems to provide good metal removal e?ciency for some metals, although it is still in its very early stage of actual applica-tion to metal removal in sludge.Studies done on cellulose seemed to indicate limitations on the complexity of the pro-cess and the cost of ligands suitable for e?ective extraction of metals.Ligands which can not be regenerated can add additional cost to the system.5.Conclusion

Chemical speciation studies showed a wide variation in the forms of metals present in sewage sludges studied, although the following forms of metals seem to predomi-nate:Cd,Cr,Cu,Pb and Ni in carbonate,organic and residual forms;while Mn is mostly in an organically bound form.It was also found that each individual metal had a characteristic fractionation pro?le independent of the type of sludge.Moreover,metals in the exchangeable,sorbed and organically-bound fraction are likely to be compara-tively mobile once disposed to land.In general,it was found that the chemical forms of heavy metals present in sewage sludges vary widely according to the nature of

the

individual metal,the characteristics of the wastewater trea-ted and the sludge treatment employed.

Experimental studies on the application of di?erent extraction technologies for removal of heavy metals from sludge revealed a broad range in metal extraction e?cien-cies,which can be attributed to di?erences in sludge compo-sition,pretreatment of sludge,forms of metals in sludge, sludge metal concentration and the extraction conditions.

For chemical extraction,the inorganic acids can extract as much as100%of some heavy metals,but at a much lower pH(1.5–2),thus,requiring more quantities of acid in the process and large quantities of water for sludge rins-ing.This makes the process more expensive because more lime would also be required to neutralize the sludge after extraction.Besides,inorganic acids are not biodegradable, which makes the process environmentally unattractive.The same is true with chelating agents and inorganic chemicals. Organic acids are more promising since at mildly acidic pH (3–4),higher metal extraction e?ciency can be achieved compared to inorganic acids at the same pH.Moreover, organic acids are readily degradable under aerobic and anaerobic conditions making the decontaminated sludge suitable for composting after dewatering.Hence,interest can be directed more toward utilizing organic acids as extracting agents for the removal of heavy metals.

The bioleaching process seems to give a higher yield of metal extraction and the chemical cost was found to be 80%lower than the chemical process because of lower acid and lime requirements.Although by using the chemical method,the Cu concentration can be reduced to an accept-able level at pH1.5or lower,in the microbial process,this metal can be decreased to an acceptable level at a compar-atively higher pH3–4.Another advantage of the bioleach-ing process is that it does not a?ect the soil conditioning and fertilizing values of the sludges.The volatile solids, nitrogen,phosphorus and potassium concentration were found to be unchanged after bacterial leaching.

Although bioleaching seemed to o?er more advantages over acid treatment,this technology is possibly limited by the inability of the system to cope with the natural environ-mental conditions since no full scale studies of the technol-ogy have been done so far.Moreover,the bioleaching process requires strict monitoring of the important param-eters such as aeration rate and temperature.In addition, the applicability of the process seems to be limited to only low solids concentration since it was observed that an increase of solid concentration decreased the metal removal e?ciency by the bioleaching process.Besides,the accumu-lation of heavy metals in the system may reach levels that can become toxic to microorganisms.

Despite good results obtained with the pilot scale studies on electroreclamation,the technology is not yet fully mature. Most of the previous laboratory studies were limited to experiments conducted on commercially available clay min-erals by arti?cially introducing various contaminants.Hence, a strong need exists to conduct further basic and applied studies before electroreclamation achieves its full potential.

The supercritical?uid extraction(SFE)method,on the other hand,is still in its very early stage of actual applica-tion to metal removal in sludge and may be limited by the complexity of the process and the cost of ligands suitable for e?ective extraction of metals from sludge.Research can be directed more on the use of ligands which can be regenerated,primarily to reduce treatment cost.

Finally,among the extraction technologies reviewed, bioleaching seemed to give higher yields of extraction for most metals but has yet to be proven applicable due to the fragility of the microbial community and to the poor performance of microorganisms under the prevailing anoxic conditions of the system.Since treatment cost data for some technologies are not available,a quantitative cost comparison of the technologies reviewed cannot be made, although some cost data revealed that bioleaching was found to cost80%less than the chemical extraction pro-cess.The common advantage for almost all treatment tech-nologies studied is that the extraction e?ciencies for some metals are high enough to remove metals from sludge, especially from agro-industries,to levels below local legal standards and,hence,suitable for land application.For developed countries where most or all of the domestic sludges are within legal standards,the goal,however,is not acceptable levels but perhaps more on the protection of soils from over accumulation of metals over extended periods of land application.

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机械制造及金属工艺试题七(含答案)

考试卷（七） 班级姓名成绩 一．选择题(20分) 1．构成工序的要素之一是（）。 A同一台机床 B同一套夹具 C同一把刀具 D同一个加工表面 2．普通车床的主参数是（）。 A车床最大轮廓尺寸 B主轴与尾座之间最大距离 C中心高 D床身上工件最大回转直径 3．.影响切削层公称厚度的主要因素是: A.切削速度和进给量； B背吃刀量（切削深度）和主偏角；C进给量和主偏角。 4．在切削平面内测量的角度有： A.前角和后角； B主偏角和副偏角； C刃倾角。 5．.精车铸铁工件时，一般： A不用切削液或用煤油； B.用切削液； C用高浓度乳化液； D用低浓度乳化液。 6．精加工夹具的有关尺寸公差常取工件相应尺寸公差的（）。 A 1/10～1/5 B 1/5～1/3 C 1/3～1/2 D 1/2～1 7．车削加工中，大部分切削热（）。 A传给工件 B传给刀具 C传给机床 D被切屑所带走 8．箱体类零件常采用（）作为统一精基准。 A一面一孔 B一面两孔 C两面一孔 D两面两孔 9．直线尺寸链采用极值算法时，其封闭环的下偏差等于（）。 A增环的上偏差之和减去减环的上偏差之和 C增环的下偏差之和减去减环的上偏差之和

B增环的上偏差之和减去减环的下偏差之和 D增环的下偏差之和减去减环的下偏差之和 10．柔性制造系统（FMS）特别适合于（）生产。 A单件 B多品种、中小批量 C少品种、中小批量 D大批量 二．判断题(10分) 1．CIMS是一种制造哲理。（） 2．夹具装夹广泛应用于各种生产类型。（） 3．定位误差是由于夹具定位元件制造不准确所造成的加工误差。（） 4．.加工塑性材料与加工脆性材料相比，应选用较小的前角和后角。 5．在切削用量中，对切削热影响最大的是背吃刀量（切削深度），其次是进给量。（） 6．磨削过程的主要特点是背向力（法向力）大，切削温度高，加工硬化和残余应力严重。（） 7．普通车床导轨在垂直面内的直线度误差对加工精度影响不大。（） 8．零件表面残余应力为压应力时，可提高零件的疲劳强度。（） 9．创成式CAPP系统以成组技术为基础。（） 10．电解加工可以加工任何高硬度的金属材料和非金属。（） 三．简答题(20分) 1．有人讲"成组技术是现代制造技术的一个重要理论基础"，如何理解这句话？2．切削液有何功用？如何选用？ 四．综合题(50分) 1．在图1所示工件上加工键槽，要求保证尺寸和对称度0.03。现有3种定位方案，分别如图b，c，d所示。试分别计算3种方案的定位误差，并选择最佳方案。

控制软件说明书

控制软件说明书 PC端软件FTM 安装及应用 系统运行环境： 操作系统中英文Windows 98/2000/ NT/XP/WIN7/ Vista， 最低配置 CPU：奔腾133Mhz 内存：128MB 显示卡：标准VGA，256色显示模式以上 硬盘：典型安装 10M 串行通讯口：标准RS232通讯接口或其兼容型号。 其它设备：鼠标器 开始系统 系统运行前，确保下列连线正常： 1：运行本软件的计算机的RS232线已正确连接至控制器。 2：相关控制器的信号线，电源线已连接正确； 系统运行步骤： 1：打开控制器电源，控制电源指示灯将亮起。 绿色，代表处于开机运行状态；橙色代表待机状态。 2. 运行本软件 找到控制软件文件夹，点击FWM.exe运行。出现程序操作界面：

根据安装软件版本不同，上图示例中的界面及其内容可能会存在某些差别，可咨询我们的相关的售后服务人员。 上图中用红色字体标出操作界面的各部分的功能说明： 1. 菜单区：一些相关的菜单功能选择执行区。 2. 操作区：每一个方格单元代表对应的控制屏幕，可以通过鼠标或键盘的点选，拖拉的方式选择相应控制单元。 3.功能区：包含常用的功能按钮。 4.用户标题区：用户可根据本身要求，更改界面上的标题显示 5.用户图片区：用户可根据本身要求，更改界面上的图片显示，比如公司或工程相关LOGO图片。 6.附加功能区：根据版本不同有不同的附加项目。 7.状态区：显示通讯口状态，操作权限状态，和当前的本机时间，日期等。 如何开始使用 1. 通讯设置 单击主菜单中“系统配置”――》“通讯配置” 选择正确的通讯端口号，系统才能正常工作。 可以设置打开程序时自动打开串口。 2.系统配置

统一认证系统_设计方案

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财政票据 网络版 电子化系统开票端操作手册

财政票据(网络版)电子化系统 开票端 操 作 说 明 福建博思软件股份有限公司

目录 1.概述 业务流程 流程说明：

1.单位到财政部门申请电子票据，由财政把单位的基本信息设置好并审核完后，财政部门给用票单位发放票据，单位进行领票确认并入库。 2.在规定时间内，单位要把开据的发票带到财政核销，然后由财政进行审核。 系统登录 登入系统界面如图： 登录日期：自动读取主服务器的日期。 所属区划：选择单位所属区划编码。【00安徽省非税收入征收管理局】 所属单位：输入单位编码。 用户编码：登录单位的用户编码【002】 用户密码：默认单位密码为【123456】 验证码：当输入错误时，会自动换一张验证码图片； 记录用户编码：勾选系统自动把用户编码保存在本地，第二次登录不需要重新输入。 填写完正确信息，点【确定】即可登入系统。 进入系统 进入系统界面如图： 当单位端票据出现变动的时候，如财政或上级直管下发票据时，才会出现此界面:

出现此界面后点击最下方的确认按钮，入库完成。 当单位端票据无变动时，直接进入界面： 2.基本编码人员管理 功能说明：对单位开票人员维护，修改开票人名称。 密码管理 修改开票人员密码，重置等操作。 收发信息 查看财政部门相关通知等。

3.日常业务 电脑开票 功能说明：是用于开票据类型为电子化的票据。 在电脑开票操作界面，点击工具栏中的【增加】按钮，系统会弹出核对票号提示框，如图： 注意：必须核对放入打印机中的票据类型、号码是否和电脑中显示的一致，如果不一致打印出来的票据为无效票据，核对完后，输入缴款人或缴款单位和收费项目等信息，全部输入完后，点【增加】按钮进行保存当前票据信息或点【打印】按钮进行保存当前票据信息并把当前的票据信息打印出来；点电脑开票操作界面工具栏中的【退出】则不保存。 在票据类型下拉单框中选择所要开票的票据类型，再点【增加】进行开票。

统一身份认证平台讲解-共38页知识分享

统一身份认证平台讲解-共38页

统一身份认证平台设计方案 1）系统总体设计 为了加强对业务系统和办公室系统的安全控管，提高信息化安全管理水平，我们设计了基于PKI/CA技术为基础架构的统一身份认证服务平台。 1.1.设计思想 为实现构建针对人员帐户管理层面和应用层面的、全面完善的安全管控需要，我们将按照如下设计思想为设计并实施统一身份认证服务平台解决方案： 内部建设基于PKI/CA技术为基础架构的统一身份认证服务平台，通过集中证书管理、集中账户管理、集中授权管理、集中认证管理和集中审计管理等应用模块实现所提出的员工帐户统一、系统资源整合、应用数据共享和全面集中管控的核心目标。 提供现有统一门户系统，通过集成单点登录模块和调用统一身份认证平台服务，实现针对不同的用户登录，可以展示不同的内容。可以根据用户的关注点不同来为用户提供定制桌面的功能。 建立统一身份认证服务平台，通过使用唯一身份标识的数字证书即可登录所有应用系统，具有良好的扩展性和可集成性。

提供基于LDAP目录服务的统一账户管理平台，通过LDAP中主、从账户的映射关系，进行应用系统级的访问控制和用户生命周期维护管理功能。 用户证书保存在USB KEY中，保证证书和私钥的安全，并满足移动办公的安全需求。 1.2.平台介绍 以PKI/CA技术为核心，结合国内外先进的产品架构设计，实现集中的用户管理、证书管理、认证管理、授权管理和审计等功能，为多业务系统提供用户身份、系统资源、权限策略、审计日志等统一、安全、有效的配置和服务。 如图所示，统一信任管理平台各组件之间是松耦合关系，相互支撑又相互独立，具体功能如下：

用友T软件系统操作手册

用友T软件系统操作手 册 Pleasure Group Office【T985AB-B866SYT-B182C-BS682T-STT18】

用 友 T+ 软 件 系 统 操 作 手 册 版本号：目录

一、系统登录 、下载T+浏览器 首次登陆需要用浏览器打开软件地址，即：(一般服务器默认设置，具体登陆地址请参考实际配置)，第一次登陆会提示下载T+浏览器，按照提示下载安装T+浏览器，然后打开T+浏览器，输入软件登陆地址。 ，T+浏览器， 、软件登陆 按键盘上的“回车键（enter）”打开软件登陆页面，如下： 选择选择“普通用户”，输入软件工程师分配的用户名和密码，选择对应的账套，以下以demo为例，如下图： 点击登陆，进入软件， 二、基础档案设置 、部门、人员档案设置 新增的部门或者人员在系统中可按照如下方法进行维护， 、往来单位设置 供应商客户档案的添加方法如下： 添加往来单位分类： 、会计科目及结算方式设置 会计科目： 系统预置170个《2013小企业会计准则》科目，如下：

结算方式，如下： 三、软件操作 、凭证处理 填制 进入总账填制凭证菜单，增加凭证，填制摘要和科目，注意有辅助核算的会计科目， 以下为点开总账的处理流程图： 如若现金流量系统指定错误，可按照以下步骤修改： 凭证在没有审核时，可以直接在当前凭证上修改，然后点击“保存”完成修改； 凭证审核 进入总审核凭证菜单下，如下图： 选择审核凭证的会计期间： 、凭证记账 进入凭证菜单下的记账菜单， 、月末结转 期间损益结转 四、日常帐表查询与统计 、余额表 用于查询统计各级科目的本期发生额、累计发生额和余额等。传统的总账，是以总账科目分页设账，而余额表则可输出某月或某几个月的所有总账科目或明细科目的期初余额、本期发生额、累计发生额、期末余额，在实行计算机记账后，我们建议用户用余额表代替总账。