Experimental Study On The Removal Of Heavy Metals From Contaminated Site Soil By Compound Washing Reagents And The Treatment Of Soil Washing Wastewater

At present,the heavy metal pollution in industrial soil was serious and posed a severe threat to the reuse of land resources,which has attracted much attention.Soil washing technology has been widely used in the remediation of heavy metal contaminated soil,and artificial chelating agents and organic acid washing agents was highly favored because of their strong ability to bind heavy metals in soil.The traditional heavy metal wastewater treatment technology such as precipitation method is invalid in the treatment of soil washing wastewater because of the stable complexed heavy metals.In this paper,the effect of EDTA-organic acids compound washing agents on the total amount of Cu,Ni and Zn in industrial contaminated soil was investigated by batch soil shock washing experiment,and the leaching toxicity of heavy metal was measured,and the changes of the speciation and stability of heavy metals were analyzed by BCR extraction.The degradation experiment of EDTA-Cu by advanced oxidation technology was conducted to explore the degradation efficiency,influencing factors and decomplexation mechanism of the clathrate.In addition,the actual soil washing wastewater was treated by the combination of advanced oxygen solution and alkali adjustment and precipitation,and the following main conclusions were obtained:?1?EDTA was mixed with citric acid,oxalic acid and tartaric acid at a certain concentration gradient,respectively,and the removal efficiency and influencing factors of Cu,Ni and Zn in contaminated soil were also explored.It was found that the removal efficiency of various heavy metals in soil was 79.7%?86.9%while 0.05 mol·L^-1 EDTA was mixed with 0.2 mol·L^-1 citric acid,oxalic acid and tartaric acid,with the solid-liquid ratio was 1:10,the washing time was 6 h and the initial p H was 3.0,which was significantly higher than that of a single washing agent.The function in accelerating desorption of heavy metals on soil surface and the ability to combine heavy metals of washing reagents was improved through the interaction between EDTA and organic acids.?2?It was found that the leaching toxicity of Cu,Ni and Zn in the soil after washing treatment was significantly reduced by toxicity extraction experiment of heavy metals.BCR extraction experiment shown that the removal efficiency of heavy metals in the weak acid extraction state?F1?were the highest in all the extraction states,which were above 90%.Therefore,the proportion of heavy metals in the weak acid extraction state in the soil after washing treatment decreased,the proportion of heavy metals in the residue state with high stability increased,and the stability of the residual heavy metals in the soil was significantly enhanced.?3?The results of degradation experiment showed that 97.1%of EDTA-Cu and69.6%of TOC could be removed after 60 minutes under optimal reaction conditions by copper-based biochar?Cu O@BC?which prepared by hydrothermal method and high-temperature calcination method activated sodium persulfate?PS?in solution,and 62.2%of copper could also be removed after alkali precipitation.The synergistic effect of Cu O and BC in activating PS was verified by the control experiments.The degradation mechanism of EDTA-Cu was illustrated by quenching results and EPR spectra:the sulfate radical(SO₄^·-),hydroxyl radical?·OH?and superoxide radical(O₂^·-)had little effect,and PS was mainly achieved by nonradical pathway with Cu O@BC.?4?The treatment of soil washing wastewater was conducted by Cu O@BC-PS catalytic system and alkali precipitation.The 81.3%of multiple clathrates?EDTA-metals?,62.2%of COD,57.8%of TOC,62.5%of Cu,75.9%of Ni and 76.9%of Zn in the soil washing wastewater were effectively removed under optimal conditions.Cu O@BC was stable and in the actual wastewater treatment process.