Effect Of Deactivation And Ecotoxicity Of Heavy Metals In Sludge By Attapulgite

Sewage sludge is an inevitable end by-product of sewage treatment. With the rapiddevelopment of cities and industry, municipal sewage emissions increased dramatically. Howto deal with the accumulation of sewage sludge become the focus problem. Municipal sludgecontains substantial N, P, K and other nutrients, hence, it is used preferentially as a fertilizerin agriculture. But the poisonous and harmful heavy metals limited the agriculturalapplication of sewage sludge, as it’s tend to accumulate through the food chain and bringpotential risks to animals and humans. Therefore, how to deal with heavy metals in sewagesludge and reduce the harm to environment is a hot research topic in the sludge utilization.In this study, sewage sludge was prepared from Lanzhou Qilihe-aning sewage treatmentplant, attapulgite clay collected from Jingyuan (county in Gansu province, China). Throughthe detection the pH and EC, the total amount of four kinds of heavy metals (Cu, Ni, Cd, Zn),water soluble state and DTPA-extractable state content, BCR Sequential extraction andecological toxicity in seven proportions of attapulgite and four time (0d,10d,20d and30d) ofattapulgite-sludge mixture, we analysised and reserched attapulgite’s influence on theeffectiveness of heavy metals in sewage sludge. The main research conclusions as follows:(1) The pH value of the sludge mixture increased with the rise of attapulgite usage, andreduced with the increasing of stacking time. The EC value of each mixed sludge decreasedwith the increasing the proportion of attapulgite. The EC value of pure sludge (CK) was stablewith the change of stacking time, the other EC value of mixture reached minimum at10d, andthen increasing, and reached maximum at30d.(2) The four kinds of heavy metal total content order for Zn> Cr> Cu> Ni, which areall lower than the national agricultural standard. The formation distribution of Cu, Ni and Cr:as follows: F4> F3> F1>F2, the formation distribution of Zn as follows: F4> F3> F2> F1.The unstable (F1, F2and F3) forms proportion of the sum of Cu, Ni, Cr and Zn were66.84%,61.53%,44.64%and48.06%, respectively.(3) Increasing the proportion of attapulgite and extending mixing time could reduce thecontent of water soluble of Cu, Ni, Cr and Zn. Extending the time of attapulgite mixed withsludge showed certain passivation on Ni, Cr and Zn, the content of DTPA-extractable ofwhich decreased, but Cu was activated. Increasing the proportion of attapulgite is advangeousto the passivation of Cu, Ni, Cr and Zn. The more content of attapulgite, the passivation wasstronger.(4) In the same time, increasing the proportion of attapulgite could promote the activeforms (F1, F2and F3) of Cu to that in stable form (F4), the exchangeable form (F1) of Ni tothat in F2, F3and F4, the F1, F2and F4of Cr to that in F3, the F1and F3of Zn to that in F2and F4. In the same proportion of attapulgite, with the extending of time, the F1and F4of Cuto that in F3, the F3and F4of Ni to that in F1and F2, the F1, F2and F4of Cr to that in F3, the F1and F3of Zn to that in F1and F2.(5) Increasing the proportion of attapulgite could reduce the mobility of Cu, Ni and Zn,but improve the mobility of Cr. Extending the mixing time of attapulgite and sludge couldreduce the mobility of Zn, but improve the mobility of Cu, Ni and Cr.(6) Increasing the proportion of attapulgite and extending the mixing time causedsignificant increase in the germination index of brassica chinensis, which has positive effecton reducing the ecological toxicity of heavy metals.