Extration Of Heavy Metals From Waterworks Sludge Using Green Organic Acids And Chelants

With the rapid growth of world economy and population,the heavy metalcontainmination of water sources, long term or in accident, is getting worse and worse,which inevitably contributes to the great increasement of heavy metals in waterworkssludge. And the metal berring sludge is becoming one of the key concentrion ofwatersupply industry as the environmental protection consciousness of bothgovernment officals and common people constantly growing.Compared withtraditional landfilling and incineration,the land application of sludge as agriculturalfertilizer or soil conditioner is more reasonable, however the high content of heavymetals in the waterworks sludge is the main limiting factor. The dewater sludge from awaterworks alongside Xiangjiang river was selected as target sludge. This study isconducted to remove and reduce typical heavy metals from waterworks sludge bychemical measure using green organic acids and chelant,such as citric acid(CTA),tartaric acid(TTA) and nitrilotriacetic acid (NTA). The effects of chelant concentration,pH, solid-to-liquid ratio, extration time, adding amount of H2O2and the extration timeof ultrasonication on the extraction of heavy metals from the sludge samples wereexamined. The speciation of metals in waterworks sludge before and after extrationwere also analysised by using BCR sequential extraction procedure.The experiment results showed that, using0.5mol/L CTA,0.5mol/L TTA and0.1mol/L NTA, with sludge-to-liquid ratio1:50and without adjustment of pH, theremoval efficiencies of Cu could reached49.1%,50.2%and58.7%, respectivelyafter24h extration at the rotation speed of150r/min under room temperature (25℃);the removal rate of Pb under the same condition were55.8%,58.2%and69.8%,respectively; the extraction rate of Cd under the same condition were up to86.2%,85.2%and88.0%, respectively; the removal rate of Cr under the same condition onlygot30.5%,29.6%%, and14%, respectively; The effects of H2O2addition andultrasonic application on Cu removal from sludge treated by chelant were mostobviously, while the impact on Pb and Cr removal was limited; While the oxidant hadlittle impact on the Cd removal, the removal rate of Cd by NTA was improved to96.2%after60min ultrasonication.The BCR sequential extraction results showed that in the sludge samples Cuwas mainly in the organic bound fraction; Pb and Cr were principally distributed in the residual fraction, suggesting relatively high stability. However, the concentration ofCd in the sludge samples surpassed the limmit according to the national sludgestandards for agricultural applicaiton in China, indicating great environment threatsince most of Cd in sludge was found in the acid extracted fraction and oxidizablefraction.Sludge sample after extraction experiments were also analysised by BCRmeasures, the results showed that the majority of Cu, Pb and Cr were in the residualfraction after treated by three different chelants individually,or with the addition ofoxidant or the ultrasoniaction, which implied high stability and low bioavailability; Cdin sludge treated by CTA and TTA was mostly found in the residual fraction, while thecontent of acid extracted fraction was found much higher in NTA treated sludge;Although the ultrasonication enhanced the process of chelants extraction of Cd fromsludge,the content ratio of Cd in acid extracted and oxidizable fraction growedobviously.