Removal Of Heavy Metals From Industrial Sludge With CETSA And IA-AA

Heavy metals have become one of the main obstacles restricting sludge agriculture,and it is one of the important topics in the field of environmental pollution prevention and control to carry out the removal and remediation of heavy metals in sludge.Scale inhibitor has attracted attention to the field of soil remediation because of its strong metal removal ability,and sludge has similar physical and chemical properties to soil.Therefore,it is worth exploring that the feasibility of using scale inhibitor to remove heavy metals in sludge.The effect of interaction between factors of heavy metal removal efficiency and the effect of washing agent on sludge properties needs to be further elucidated.In this study,carboxyethylthiobutyric acid（CETSA）and itaconic-acrylic acid copolymer（IA-AA）were used as eluent materials to systematically analyze their effects on the removal of heavy metals in sludge and on the properties of sludge,providing an environmentally friendly technical scheme for the subsequent rational utilization of sludge.The main results are as follows:（1）CETSA and IA-AA have good washing effect on nickel（Ni）,lead（Pb）,zinc（Zn）,cadmium（Cd）,and chromium（Cr）.The removal rates increased significantly（P<0.05）with the increase in washing solution concentration(10-100 g L^-1)and washing time（10-1440 min）,with the decrease of pH（2-8）and solid-liquid ratio（1:10-180）（P<0.05）.Kinetic studies suggested thatthe removals of five heavy metals with CETSA and IA-AA washing were fitted well with pseudo-second-order kinetic models.（2）Response surface optimization（RSM）results showed that washing solution concentration was the main factor to control CETSA remove heavy metals,followed by pH of washing solution and washing time.Nevertheless,pH of washing solution was the main factor to control IA-AA remove heavy metals,followed by washing time and IA-AA concentration,and there is a significant interaction effect（P<0.05）between pH of washing solution and IA-AA concentration.The optimum washing parameters of heavy metals were obtained by the optimization model,and the total removal rate reached 68.63%and 97.47%in this condition,respectively.Finally sludge treated reached the B and A grade standards of Control stanards of pollutants in sludge for agricultural use.（3）The main mechanisms of heavy metal washing removal of sludge are acid dissolution,electrostatic adsorption and chelation reactions.As the washing solution was added,the protonation（H⁺）in the solution was severe,the metal was dissolved and desorbed from the sludge colloid.At the sametime,the surface of the sludge particles was positively charged and reduce their adsorption force to heavy metals,the dissociated metal ions inter attraction with the carboxyl or sulfonic groups of the solutiont due to coulombic force.In addition,the exchangeable heavy metals were soluble in acidic conditions and chelat with the carboxyl and sulfonic groups in the solution.（4）The easier extractable components（exchangeable and reducible）of heavy metals were effectively removed after CETSA and IA-AA washing,then the heavy metals components of sludge are mainly stable forms of oxidation state and residue state.They have poor transportability and low bioavailability,which reduce the risk of sludge land use to some extent.（5）The washing process will have different effects on the environmental quality of sludge.The pH and alkali-hydrolyzed nitrogen（AN）content in the sludge decreased 0.26-0.81 units and 4.7-24.8%after washing.The total nitrogen（TN）and available phosphorus（AP）after CETSA treatment were decreased but less by 1.21%and 5.75%,respectively.There were no significant changes in total nitrogen（TN）,total phosphorus（TP）,and total potassium（TK）contents after IA-AA treatment,as well as total phosphorus（TP）and available potassium（AK）after CETSA washing（P>0.05）.The content of organic matter（SOM）increased by 6.9-14.4%.Available phosphorus（AP）content after IA-AA and total potassium（TK）after CETSA also increased 6.42%and 10.79%,respectively.（6）The Fourier transform infrared spectroscopy（FT-IR）of sludge before and after CETSA and IA-AA washing showed that some characteristicpeaks position displacement and peak strength weakening,which is negligible,indicating the washing have a certain impact on the chemical environment of the sludge system,but have not caused the destruction of the sludge structure.Scanning electron microscopy and energy spectrum（SEM-EDS）analysis confirmed there were only minor effects on the sludge morphology and skeleton.The sludge aggregates showed independent small particle material dispersion distribution and the particle profiles not clear after CETSA washing.There was no obvious change in the overall shape of the original sludge,but the structure was more loose and the surface was more smooth after IA-AA washing.The concentration of some elements in sludge decreased significantly,while the weight percentage of C increased significantly.The results of X ray diffraction（XRD）showed that CETSA and IA-AA have little effect on sludge during washing process.