不同锆负载量锆改性膨润土对水中磷酸盐吸附作用的对比

通过实验对比考察了不同锆负载量的锆改性膨润土对水中磷酸盐的吸附作用.结果表明,锆改性膨润土对水中磷酸盐的吸附动力学过程符合准二级动力学模型,整个过程可以分为快速吸附阶段、缓慢吸附阶段和平衡吸附阶段,其中缓慢吸附阶段的吸附速率受膜扩散和颗粒内扩散所控制.锆改性膨润土对水中磷酸盐的吸附等温实验数据可以采用Langmuir、Freundlich、Sips和Dubinin-Radushkevich等温吸附模型进行拟合.实验条件下,磷酸盐吸附性能随pH增加而降低.溶液共存的Na~+、K~+和Ca~(2+)促进了锆改性膨润土对磷酸盐的吸附,并且Ca~(2+)的促进作用远远大于Na~+和K+,而溶液共存的HCO-3和SO2-4一定程度上抑制了锆改性膨润土对磷酸盐的吸附.锆改性膨润土吸附水中磷酸盐的主要机制为配位体交换并形成内层磷酸盐配合物.锆改性膨润土对水中磷酸盐的吸附能力随着锆负载量的增加而增加,而锆改性膨润土中单位质量ZrO_2对水中磷酸盐的吸附量则随着锆负载量的增加而降低.当ZrO_2负载量由3.61%增加到13.15%时锆改性膨润土的最大单层单位吸附量(以P计)显著地由3.83 mg·g~(-1)增加到9.03 mg·g~(-1),而继续增加ZrO_2负载量至19.63%时锆改性膨润土的最大单层单位吸附量则缓慢地提高到9.66 mg·g~(-1)(以P计).当ZrO_2负载量由3.61%逐渐增加到19.63%时,锆改性膨润土中单位质量ZrO_2的磷酸盐最大吸附量m(P)/m(ZrO_2)]由106 mg·g~(-1)逐渐下降到49.2 mg·g~(-1).综合考虑吸附剂的经济成本和吸附容量,ZrO_2负载量为13.15%锆改性膨润土更为适合作为吸附剂去除水中磷酸盐.

Comparison of Phosphate Adsorption onto Zirconium-Modified Bentonites with Different Zirconium Loading Levels

In this study, zirconium-modified bentonites (ZrMBs) with different zirconium loading levels were prepared, and the adsorption behaviors of phosphate on these ZrMBs were comparatively investigated using batch experiments. The results showed that the kinetic process of phosphate on ZrMBs well followed the pseudo-second-second kinetic model. The kinetic process was divided into three stages, including a rapid external surface adsorption stage, a gradual adsorption stage where both the intra-particle diffusion and film diffusion were rate-controlled, and a final equilibrium adsorption stage. The equilibrium adsorption data of phosphate on ZrMBs could be well described by the Langmuir, Freundlich, Sips and Dubinin-Radushkevich isotherm models. Phosphate adsorption onto ZrMBs was more favorable under strongly acidic condition than under weakly acidic or neutral condition, while phosphate adsorption onto ZrMBs under weakly acidic or neutral condition was more favorable than that under alkaline condition. Coexistence of Na⁺ and K⁺ slightly enhanced phosphate adsorption onto ZrMBs, while coexisting Ca²⁺ greatly enhanced the phosphate adsorption. The presence of HCO₃^- or SO^2-₄ inhibited the adsorption of phosphate on ZrMBs. The mechanism for phosphate adsorption onto ZrMBs followed the ligand exchange and inner-sphere complexing mechanism. The phosphate adsorption capacity for ZrMB increased with increasing loading level of zirconium, while the amount of phosphate adsorbed on unit mass of ZrO₂ in ZrMB decreased with increasing loading amount of zirconium in ZrMB. When the loading amount of ZrO₂ in ZrMB increased from 3.61% to 13.15%, the maximum phosphate adsorption capacity (MPAC) for ZrMB increased from 3.83 to 9.03 mg·g^-1, while a further increase in the ZrO₂ loading amount to 19.63% resulted in a slight increase of MPAC to 9.66 mg·g^-1. However, an increase in the loading amount of ZrO₂ in ZrMB from 3.61% to 19.63% caused a decrease of the MPAC for the ZrO₂ located in ZrMB from 106 to 49.2 mg·g^-1. Considering both cost and adsorption capacity of adsorbent, the ZrMB with 13.15% of zirconium loading amount could be more suitably used as an adsorbent to remove phosphate from aqueous solution than the other ZrMBs.