稻田土壤镉的表面络合模型及其生物有效性验证

Surface complexation model of Cd in paddy soil and its validation with bioavailability

中国南方分布的稻田土壤以酸性可变电荷土壤为主(pH <6.5).重金属Cd作为重要污染物,其生物有效性在酸性条件下相对较高,因此,研究可变电荷稻田土壤中Cd的吸附特征并建立模型准确评价Cd的生物有效性具有重要意义.本研究通过测试不同p H下的稻田土壤Cd吸附曲线,建立了表面络合模型(SCM),模拟Cd的吸附行为;并且采用田间实验的稻米Cd累积量,对SCM预测的有效态Cd进行验证. Cd的吸附实验表明,随pH升高Cd的吸附量升高,至pH 5.5时Cd以吸附态为主.利用电位滴定实验和不同pH下Cd的吸附实验,建立了Cd吸附的1-site/2-p K表面络合模型,该模型能够很好地拟合Cd在可变电荷土壤中随p H变化的吸附特征.水稻田间实验数据表明,酸性(pH <5.5)稻田土壤pH与水稻籽粒Cd呈显著负相关(P <0.05),因为p H决定的吸附平衡是影响有效态Cd含量的关键;采用连续提取法获得的Cd形态(1 mol L^-1MgCl2提取的可交换态和1 mol L^-1NaOAc/HOAc提取的碳酸盐结合态)与水稻籽粒Cd含量无显著相关(P> 0.05);而采用模型预测的溶解态Cd与水稻籽粒Cd含量极显著相关(P <0.01),说明本模型预测的有效态Cd明显优于连续提取法.因此,应用SCM模型,可以更为准确地预测稻田土壤有效态Cd及其Cd在土壤-水界面的形态分配,为Cd的生物有效性评估提供新的思路.

In southern China, the paddy soils are mainly variable-charge soils, which have undergone high weathering processes and leaching processes. Due to deficiencies of carbonate and base cations, the variable-charge soil pH is normally less than 6.5,and decrease of soil pH can substantially enhance the mobility and bioavailability of heavy metals. Cadmium(Cd) is one of the most important pollutant influencing human health and environmental safety. Cd contamination is a serious problem in paddy soils, and the acidic condition could further aggravate the mobility and bioavailability of Cd. There is an urgent need to establish a model to investigate the adsorption characteristics of Cd and accurately assess the bioavailability of Cd in variable-charge paddy soils. In this study, the paddy soil was selected from agricultural regions in southern China. Batch experiments were conducted to investigate the Cd adsorption characteristics of the soils from pH 3 to 10. The results showed that Cd mainly existed in dissolved forms at pH < 4.0. With increasing pH, the concentration of dissolved Cd decreased but the adsorbed Cd increased. At pH > 5.5, almost all Cd was adsorbed on the soil surface. Based on the potentiometric titration, the point of zero charge(pHpzc) was obtained when the surface charge density(σ) was equal to zero. The p Hpzcof the paddy soil was 5.91. The 1-site/2-p K surface complexation model(SCM) was applied to further examine the acid-base properties of the soils by assuming the acid-base buffering system as a protonation-deprotonation process. The surface site concentration(Hs) and the surface site density(Ds) were obtained by Gran plot. The two acid equilibrium constants(p Ka1 and p Ka2) were determined by extrapolation and the calculated pHpzcwere obtained by the average values of the p Ka1 and p Ka2. The value of pHpzccalculated from the model was 6.10, which was well matched with those from the potentiometric titration, indicating that it is feasible for the application of SCM to the variable-charge soils.The intrinsic equilibrium constants(lg KSOCd) was estimated from the Cd adsorption experimental data and acid-base parameters(p Ka1, p Ka2 and Ds) using Visual MINTEQ software. The results showed that the adsorption of Cd on the paddy soil could be successfully modeled by the SCM. Based on the previous study, the pH was significantly negatively correlated with grain Cd content in paddy soil when p H was below 5.5(P < 0.05), indicating that pH-dependent adsorption equilibrium was the main factor influencing the Cd bioavailability. The Cd in soils was sequentially extracted with1 mol L^-1 MgCl2(exchangeable fraction) and 1 mol L^-1 NaOAc/HOAc(bound to carbonate). Although the extracted Cd was positively correlated with Cd content in grain, the P value was much higher than 0.05, indicating the insignificant correlation. Compared with the extraction method, the dissolved Cd calculated from the SCM was significantly correlated with the grain Cd content(P < 0.01), suggesting that the model-derived Cd species was much better than the extracted Cd for reflecting the Cd bioavailability in paddy soil. Hence, this study provides a new approach to investigate the adsorption characteristics of Cd in paddy soil which could be used to accurately predict the Cd bioavailability in variable-charge soils.