The Removal Method Of Cd And As From Municipal Sludge By Dissolved Organic Matter And Nitric Acid Combining With Ultrasound

Sludge,as a by-product of wastewater biological treatment,is increasing producteed with the economic development and urbanization process,thus the rational disposal and treatment of sludge is a difficult and hot issue in current sludge.Landfill,incineration and land application are the common methods for sludge disposal.Land application is regarded as the reasonable sludge disposal method because amounts of N,P and organic matter are contained in sludge which could supply nutrition for plants and improve soil physical and chemical properties.However,a large of heavy metals in the sludge become a limitation for sludge land application.At present,the main methods for removal of heavy metal in sludge include chemical leaching,electrokinetic remediation,phytoremediation and microbial filtration.However,these are some constraints on the removal efficiency of heavy metal,treatment period and cost in these methods.In order to explore some economical and effective methods on the heavy metals removal,this paper studied the removal of Cd and As which levels in the sludge are exceeding the standard for agricultura by using dissolved organic matter(DOM)released from ultrasonic lysis of sludge and the nitric acid combined ultrasound and further explained the relevant removal mechanism.It was mainly investigated that the Cd and As removal efficiencies under different DOM concentrations,pH value,shaking time and washing times,then evaluated the leaching characteristics of Cd and As in sludge by different ultrasonic conditions,and also explored the effect of nitric acid combined with ultrasound on the removal of Cd and as in sludge.Further,the changes of Cd and As speciation in sludge were analyzed by using continuous extraction of Tessier.Research results showed that,(1)Under optimal conditions,the removal efficiencies of Cd and As were respectively 36.12% and 23.75% with single washing under DOM concentration 1200mg/L,pH 6,leaching time 24 h and the ratio of liquid to solid 40:1,and they were further improved respectively to 62.35% and 39.45% with triple washing.After triple leaching,the content of organic matters and total nitrogen and total phosphorus increased,but total potassium and pH decreased in the sludge.The fractions of Cd and As distribution in sludge was changed,and the available contents of Cd and As was significantly reduced in sludge after triple washing.(2)The dissolution rate of Cd and As from sludge reached to respectively 42.35% and58.63% under the conditions of 1.2 W/mL/30 min.The mass fraction of Cd and As in sludge were reduced from 6.80 mg/kg to 5.61 mg/kg and from 91.13 mg/kg to 54.66mg/kg.However,the mass fraction of Cd and As in sludge further reduced to 4.25mg/kg and 48.16 mg/kg,respectively,by pretreating 0.04 mol/L nitric acid combined with ultrasound,which fully met the control standards for pollutants in agricultural sludge in China(Cd ? 5mg/kg,As ? 75mg/kg).Meanwhile,the mass fraction of exchangeable,bound to carbonates and bound to Fe/Mn oxides for Cd and As significantly declined,while the fraction of th organic bound and residue bound obviously increased.