柠檬酸-施氏矿物复合体对Cd和Pb的吸附研究

施氏矿物(Schwertmannite)是酸性矿区废水污染环境中典型的铁硫酸盐次生矿物,由于其较好的吸附能力成为矿区多种重金属的沉积库.环境中同时广泛存在着各种有机质,易与矿物结合形成有机质矿物复合体,从而影响矿物对重金属的吸附行为.目前关于有机质与施氏矿物形成的复合体对重金属的吸附研究相对较少.因此,本研究选取柠檬酸作为天然有机质的代表,研究不同环境条件下柠檬酸施氏矿物复合体对重金属Cd、Pb的吸附.结果表明,复合体保持着施氏矿物基本的矿物学特征,对重金属Cd和Pb的吸附行为表现出不同的趋势:复合体对Cd的吸附量整体上随着Cd浓度和pH值的升高而上升;相反,复合体对Pb的吸附量均比纯施氏矿物要低,并且随着柠檬酸含量的升高吸附量下降越明显;体系中共存的硫酸根离子(SO₄^2-)抑制了复合体对Cd的吸附,而对Pb的吸附则起到促进作用.XRD分析表明柠檬酸抑制了高pH下Pb在矿物表面形成PbSO₄沉淀.FTIR红外谱图进一步证明结合在复合体上的柠檬酸参与了对Cd和Pb的吸附过程.本研究结果有助于更好地预测Cd和Pb在矿区的迁移和归宿.

Cd, Pb adsorption on citric acid-Schwertmannite complexes

Schwertmannite (Sch), a typical secondary iron-hydroxysulfate mineral in acid mine drainage (AMD)-contaminated environment, is a good absorbent for many heavy metals due to its high adsorption capability. Natural organic matters commonly coexist with the mineral and form organo-mineral complexes, thus affecting the toxic heavy metal adsorption process on original mineral. To date, research about the adsorption of heavy metal on organo-Sch complex remains scarce. In this study, we selected citric acid (CA) as a representative natural organic matter to examine the adsorption of Cd and Pb on different citric acid-schwertmannite complexes (CA-Sch). The results showed that the CA-Sch complexes maintained the mineralogical characteristics of Sch and exhibited different trends of adsorption for Cd and Pb: the adsorption capacity of the complexes for Cd increased with the increase of Cd concentration and pH. Conversely, citric acid had an inhibited effect on the adsorption of Pb on the organo-complex under different Pb concentrations and pH, and this effect was enhanced with the increasing citric acid concentration. The coexisting sulfate in the system inhibited the adsorption of Cd on CA-Sch, while promoted the absorption of Pb. The XRD analysis showed that CA inhibited the formation of PbSO₄(s) on Sch. The FTIR results further verified that the CA was involved in the adsorption process of Cd and Pb. These findings provide a better prediction of the mobility and fate of Cd and Pb in the AMD-contaminated environments.