| To explore the removal process and result of heavy metals by saponin and citric acid, sewage sludge collected from municipal wastewater treatment plants in wuhan and kaolinite adsorbed with Cu/Zn were conducted to investigate the removal of heavy metals by saponin and citric acid. Various factors such as the concentration of desorbents(0-7%/0-2 mol/L), pH(2-6), contact time(0-48 h), temperature(10, 25, 40 ℃), extract times(1-4) and the change of distributions of heavy metals before and after removal were studied. To explore the removal mechanism by means of the pH change of eluent during the removal and infrared spectrum analysis(FTIR). Results showed that:Heavy metals in sewage sludge could be effective removed by saponin, the removal rate of heavy metals increased with the increase of saponin concentration. The pH change had no significant effect on removal of Cu, Ni, Pb, but higher removal rate of Zn obtained when the pH decreased to 2. Cu, Ni, Pb were able to achieve the maximum removal rate when the reaction time was during 6-12 h, however, a better removal of Zn need to go through to 48 h. With the increase of temperature, the removal efficiency of Cu decreased but that of Zn increased, while Ni and Pb remained relatively constant. Cu, Ni, Pb and Zn obtained the highest cumulative removal rate of 33%、76%、21% and 39% after extracted 4 times.By single removal, the removal efficiency of heavy metals in sewage sludge by citric acid was better than saponin. The removal rate of heavy metals increased rapidly to the maximum followed by a decline with the increase of saponin concentration. With the increase of pH, the removal efficiency of Cu and Zn increased while that of Ni and Pb decreased. The maximum removal achieved only when the reaction time was during 1-6 h. With the increase of temperature, the removal efficiency of Cu increased, while Ni, Pb and Zn remained relatively constant. Cu, Ni, Pb and Zn obtained the highest cumulative removal rate of 64%、40%、79% and 81% after extracted 4 times.The removal of heavy metals on kaolinite by saponin indicated that, the removal rate of Cu and Zn increased with the increase of saponin concentration, the maximum removal rate of Cu and Zn were 50% and 49%. Higher removal rate of Cu and Zn obtained when the pH was lower. The maximum removal achieved only after 2 h. The removal efficiency of Cu was better at low temperature, while Zn remained relatively constant when the temperature changed.The removal of Cu and Zn on kaolinite by citric acid indicated that, the variation of removal rate with concentration, pH and reaction time were same as that of sewage sludge, the maximum removal rate of Cu and Zn were 56% and 49%. But when temperature changed, the removal efficiency of Cu remained relatively constant while Zn was better at high temperature.After the removal of sewage sludge and kaolinite by citric acid and the removal of kaolinite by saponin, the pH of eluent remained relatively stable. But the pH of eluent had significant effect with removal conditions when the removal of sewage sludge by saponin, with the increase of contact time, temperature, extract times and the decrease of pH value of saponin solution, the pH of eluent decreased.In the original sewage sludge samples, Cu was mainly in the presence of oxidizable and residual, Ni and Pb were mainly in the presence of residual, while that of Zn was in the presence of acid-soluble and residual. After the removal by saponin and citric acid, Cu and Ni were mainly in the presence of residual and oxidizable, the proportion of acid-soluble and reducible of Pb were increased, the proportion of residual of Zn was decreased by saponin extract while that of Zn was increased by citric acid extract.FTIR results showed that the intensity of the peak changed at the same position but changed in different level after the removal by saponin and citric acid. The heavy metals in interlayer structure and ttat combined with chemical bond could all be removed by saponin and citric acid. |
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