| Municipal sewage sludges(S1-S4)collected from municipal wastewater plants in Wuhan city were studied by the oscillation leaching experiments with combination of saponin and citric acid to evaluate their impact on removal efficiency of Cu?Ni?Pb?Zn?Cd and Cr under different volume ratio(20:1-1:20)?ratio of solid to liquid(1:20-1:80)?pH(2-6)?reaction time(0-2880min)and leaching times(1-4)and reaction time(0-2880).The fractions changes of heavy metals before and after leaching were analyzed by BCR continuous extraction method.The effects of leaching on the stability and mobility of heavy metals were investigated by calculating the stability factor(IR)and mobility factor(MF)values.Geo-accumulation index(Igeo)was used to evaluate the pollution degree of heavy metals before and after leaching.The leaching toxicity of sludge before and after leaching was analyzed,and the effects of heavy metal on germination and root elongation of Chinese cabbage were analyzed.So as to provide a theoretical reference for the effective removal of heavy metals in sludge and reuse the water for further use.Results showed that:The removal rate was increased with reducing the volume ratio of saponin and citric acid,with the maximum removal rate observed in the ratio 5:1-1:5.Compared with the volume ratio,with the reduction of solid-liquid ratio,heavy metal removal rate increased even greater.pH has a great effect on the removal of heavy metals,the removal of Cu,Ni,Zn,Cd and Cr decreases with the increase of pH,while Pb is opposite to that of other heavy metals.At pH 2-3,most of the heavy metal can achieve better removal effect.On the whole,the combined leaching of the removal effect is better than a single leaching.Saponins and citric acid combined with leaching,the removal efficiencies of Ni,Zn and Cd in 60-720 min was better,while removal efficiencies of Cu,Pb and Cr were best at 1440 min.The maximum removal rate of the heavy metals reached at 1440 min.The Elovich equation was used to fit the kinetics of heavy metal removal in sludge,by comparing the size of the parameter b in the fitting results,it was found that the removal rate of Zn was the highest and the removal rate of Cd was the lowest.A single leaching removal effect is limited,increase the number of leaching,heavy metal removal rate was significantly improved.The maximum removal rate of heavy metal for single leaching was 71.03%(Zn)and the lowest removal rate was 16.63%(Ni).After 4 times leaching,the removal rate of heavy metals increased significantly,the maximum increase of 4.48 times(Cr),the minimum increase of 1.15 times(Zn).The maximum cumulative removal rates of Cu,Ni,Pb,Zn,Cd and Cr were 78.89%,55.51%,81.73%,84.79%,56.71% and 80.77% respectively.After a single leaching,the removal rates of heavy metal in acid soluble and reducible fraction were high,while the oxidation and the residual fraction were relatively low.With increasing the number of leaching,the removal rate of various forms of heavy metals increased in different degrees.After 4 timens leaching,most of the heavy metal acid soluble and reducible fraction is almost completely removed,and the removal rates of the oxidized and residual fraction were also increased significantly,the highest increases were 4.69 times(Cr)and 9.32 times(Pb).Leaching changed the stability and mobility of heavy metals,as the stability was increased and mobility was decreased.With the increase in the number of leaching,this trend was more obvious.The accumulation index indicated that Cu and Zn were in moderate concentration during,and Cd concentration was high before leaching.After 4 times leaching,heavy metal Igeo index significantly reduced.The leaching concentration of heavy metals after leaching was greatly reduced,Where Ni was the largest decrease,the decrease of Cd wass the smallest,the leaching concentration of heavy metals after leaching was within the control standard.The germination and root elongation in Chinese cabbage showed that the toxicity of heavy metals decreased after leaching.After the sludge was leaching,the seed germination rate of cabbage was more than 90% at 20% sludge application rate,and the germination rate was about 60% in 100% sludge application rate.Under the 20% sludge application rate,the root elongation inhibition rate of cabbage was only 22.14%.The recovery rate of heavy metals increased with the increase of dose of hydrated lime.When the amount of hydrated lime was 3.2g/L,the maximum recovery rate of heavy metals is reached.The maximum recoveries of Cu,Ni,Pb,Zn,Cd and Cr were 48.77%,53.40%,95.33%,98.37%,97.23% and 83.98%,respectively. |
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