Passivation Of Heavy Metals In Municipal Sludge By Modified Attapulgite And Its Environmental Safety Assessment

Due to the rapid increase of sewage treatment capacity and treatment rate in China,the treatment and disposal of sludge has been widely concerned.Land use of sludge,especially its application in landscaping,can solve the problem of sludge treatment and disposal.Sludge used in landscaping can also improve soil fertility,promote plant growth,and have a positive impact on sustainable development.The unique structure of attapulgite makes it have better adsorption capacity and ion exchange capacity.Modified attapulgite can further improve its adsorption capacity.In this paper,modified attapulgite soil was used as passivator to treat sludge.The safety of passivated sludge applied to soil was evaluated by seed germination and root elongation experiments and pot experiment,and the pollution risk of passivated sludge applied to soil was evaluated by leaching experiment.The main results are as follows:1.The adsorption property of attapulgite was enhanced after modification.The adsorption capacity of Cu²⁺by original attapulgite is 0.31 mg/g.Through single factor experiment and orthogonal experiment,it is found that the adsorption capacity of attapulgite is 0.96 mg/g when the stirring time is 90 min,the modification temperature is 90℃,and the concentration of sodium hydroxide is 5 mol/L.Compared with the original attapulgite,the adsorption capacity is increased by 68%.2.It is feasible for the sludge to be used for landscaping after passivation.The indicators such as conductivity,p H,organic matter content and heavy metal content all meet the requirements of the sludge landscaping,and it is more suitable for the passivation sludge to be applied to alkaline soil.3.The amount of passivating agent and passivating time have great influence on the morphology of heavy metals.With the increase of passivation time and passivating agent,the residual states of the four heavy metals show an increasing trend,and their stability is enhanced.When modified attapulgite with a mass fraction of 25%was added to the sludge and passivated for 28 days,the passivation effects of the four heavy metals were all good.Under this condition,the stability coefficients of Cu,Zn,Pb and Cr decreased greatly,indicating that the passivated sludge with modified attapulgite is an effective method to reduce the activity of heavy metals.4.Passivated sludge can provide more nutrients for ryegrass growth.In the experiment of seed germination and root elongation,the relative root elongation and germination index of control treatment（CP）were 82%,and that of passivation treatment（ATP）was 98%.The growth of ryegrass was affected by the addition of sludge,and the relative root elongation and germination index of unstable treatment（NSS）were 48%and 35%,respectively.5.The ecological toxicity of sludge is reduced after passivation.In pot experiment,plant height,tiller number,wet weight,dry weight and chlorophyll content under ATP treatment were 1.2 times,2 times,1.3 times,1.3 times and 1.2 times of CP treatment,respectively.6.After sludge passivation,some heavy metals were fixed,and bioavailability was reduced.The ability of ryegrass to enrich heavy metals from the root was weakened,and the ability of ryegrass to transport heavy metals upward was also weakened.The ability of Cu and Zn to enrich and transport heavy metals was stronger than that of Pb and Cr.7.The leaching experiment confirmed that applying 10%passivated sludge to the soil would not cause pollution to groundwater,and the heavy metal content of the leaching solution was all lower than the IV limit of groundwater.The cumulative release of heavy metals and nutrients under leaching conditions is a complex process controlled by multiple factors.