Mechanism Of Phosphate And Citric Acid Affecting Pb~2+Adsorption By Red Soil Colloids

According to the mechanism of interaction between phosphorus and Pb to transform the bioavailable Pb into Pb-phosphate minerals, an alternative approach to remediate Pb-contaminated soils has been widely accepted. Phosphate rock (PR), apatite and monopotassium phosphate have been shown to effectively immobilize heavy metals in contaminated soils. Phosphate addition inducing various sorption processes or precipitation and co-precipitation contribute to reducing Pb mobility. Low molecular weight organic acids (LMWOAs) abundant in natural soils can improve phosphorus availability and influence immobilization of heavy metals in soils. They could not only enhance mobility of heavy metal through chelation but also induce immobilization of heavy metal by activating phosphorus. This study was conducted to investigate the effects of phosphorus and citric acid on Pb2+adsorption by soil colloids. And the affecting mechanisms of joint phosphate and citric acid on Pb2+adsorption were reasoned through analyzing Zeta potential and X-ray diffraction. Meanwhile we took simulating Pb contaminated soil as material, phosphate and citric acid as remediation matter, and adopted BCR continuous extraction,0.01mol/L CaCl2and toxicity characteristic leaching procedure(TCLP) to investigate the change of different forms and to evaluate the remediation effect. The purpose is to provide theoretical foundation for repairing Pb-contaminated soils using P amendment with taking LMWOAS into account. Results showed that:1. Effect of citric acid and P concentrations on Pb adsorption by red soil colloidPb adsorption capacity by red soil colloid, goethite and kaolinite in the control (PO) showed a change of peak-like curve with the increase of citric acid concentration; the citric acid concentration≤0.5mmol/L,≤5.0mmol/L and≤1.0mmol/L could promote the Pb absorption of red soil colloid, goethite and kaolinite respectively, while the citric acid concentration higher than this value represented a suppression against the Pb absorption. The relative high or low concentration of promotive or inhibitive effect of citric acid on adsorption of Pb related to mineral types. The existence of P and0.1mmol/L citric acid could promote the Pb absorption, while the citric acid concentration was0.5mmol/L represented a inhibitive effect. Both the0.1and0.5mmol/L citric acid could promote the Pb absorption. Namely,0.5mmol/L citric acid is the relative low concentration for goethite and kaolinite; while it is the relative high concentration for red soil colloid, which could be critical point of promotive or inhibitive effect.2. Secondary adsorption and desorptionPb adsorption capacity by red soil colloids in CK treatment increased with the initial P concentration; For kaolinite, the change tendency of Pb adsorption is similar with red soil colloid. For goethite, the Pb adsorption capacity firstly increased with the rising P initial concentration, and then decreased. This might be due to formation of iron-phosphorus "package" in the surface of goethite. Generally speaking, electrical adsorption ion can be exchanged by neutral salt. The desorption percentage of Pb in red soil colloid and two minerals was decreased with increasing lead sorption capacities, which indicated that the lead immobilization in red soil colloid was specific adsorption due to phosphorus and citric acid adsorbed in soil or was the generation of phosphorus and lead coprecipitation, rather than simple electrical adsorption.3. Effect of pH on Pb adsorption by red soil colloidsPb adsorption amount by red soil colloids increased with the increase of pH in the range of3to6. In the1.0mmol/L P treatment, Pb adsorption capacity increased significantly with the increase of pH from3to4, and did not increase obviously with the further increase of pH. In the0.5mmol/L citric acid treatment, the Pb adsorption capacity firstly increased with rising pH, then the adsorption capacity decreased with the increasing pH. The trend was consistent with the citric acid adsorption capacity by variable charge soil. For the combined treatment of1mmol/L P and0.5mmol/L citric acid, within all test pH ranges, the Pb adsorption capacity was higher than that of CK and citric acid treatment, but lower than that of the1.0mmol/L P treatment. It related with collaborative adsorption of P and Pb in surface of red soil colloid.4. Effect of P and citric acid on zeta potentials of red soil colloidsBoth P and citric acid could decrease zeta potential of red soil colloids. And the sequence of decreasing extent was1.0mmol/LP+0.5mmol/L citric acid>0.5mmol/L citric acid>1.0mmol/L P. For the1.0mmol/L P treatment, the zeta potential had the lowest decrement and the highest Pb adsorption capacity. This proved that, the adsorption of Pb by red soil colloids in the presence of P was not a simple electrical adsorption but a synergistic effect between Pb and phosphorus on red soil colloids surface by special adsorption, generating co-precipitation or P-Pb substance. The same as red soil colloid, P and cit treatments also could decrease Zeta potential of goethite and kaolinite.5. X-ray diffraction analysisPyromorphite was generated during Pb adsorption in1.0mmol/L P treatment, but in CK,0.5mmol/L citric acid and1.0mmol/L P+0.5mmol/L citric acid treatments, there was no obvious pyromorphite found, i.e., the mechanism that the presence of1.0mmol/L P improved the Pb adsorption by red soil colloids was that the surface co-adsorption could generate P-Pb minerals, while the existence of0.5mmol/L citric acid could inhibite the generation of such P-Pb minerals or reduced their stability.6. Evaluation of the effects and stability of phosphorus and citric acid immobilizing simulated Pb contaminated soil(1) In the absence of citric acid, the amount of acid extracted Pb was decreased along with the increase of P concentration. When P concentration was100and400mg/kg, acid extractable Pb increased with the increasing of citric acid concentration.The residual lead changed in the opposite direction from acid extractable Pb. It indicated that P declined the bioavailability of Pb, while citric acid had the opposite effect.(2) With a certain organic acid concentration, extractable Pb amounts by both CaCl2and TCLP decreased with the increasing of P concentration. But in a certain P concentration, extractable Pb contents by CaCl2and TCLP changed in the opposite trend with the increasing of citric acid concentration. Taking BCR speciation of Pb into account, TCLP extraction method was more suitable for evaluation of Phosphorus and citric acid immobilizing simulated Pb contaminated soil.