SDBS/Na+对红壤胶体悬液稳定性的影响

土壤中普遍存在的有机污染物和无机离子与土壤胶体颗粒的相互作用深刻地影响着土壤中一系列物理、化学和生物学过程.选取红壤胶体作为对象,利用动态光散射技术研究了不同浓度十二烷基苯磺酸钠(SDBS)和Na+作用下红壤胶体颗粒的凝聚过程,并结合体系的pH和Zeta电位分析了SDBS/Na+与土壤胶体颗粒相互作用的机制.结果表明:①相同浓度Na+作用下,随着SDBS浓度的升高,土壤胶体悬液稳定性增强.例如120 mmol·L-1Na+作用下,随着SDBS浓度从0 mmol·L-1升高到10 mmol·L-1,凝聚体有效粒径从702 nm下降至193 nm,总体平均凝聚速率从28.6 nm·min-1减小到3.36 nm·min-1;②相同浓度SDBS作用下,随着Na+浓度的升高,体系Zeta电位绝对值显著降低,凝聚体有效粒径逐渐增大,凝聚速率逐渐加快;③仅SDBS作用下,随着SDBS浓度的升高,体系Zeta电位绝对值从47.6 mV增加到62.2 mV,体系pH从6.17升高到6.76,但均小于土壤胶体悬液本身的pH(6.89).因此,SDBS通过疏水作用和静电作用吸附于土壤胶体颗粒表面,增加了颗粒表面的负电荷数量,降低了作用于胶体颗粒表面的有效Na+浓度(SDBS疏水长链的空间阻碍和高浓度SDBS所形成的胶束结构对Na+的吸附),使得胶体悬液稳定性增强,需要添加更多的Na+才能发生凝聚.

Impact of SDBS/Na+ on Red Soil Colloidal Stability

The interactions between soil colloidal-sized particles and organic contaminants or inorganic ions profoundly affect numerous soil physical, chemical and biological processes. The coupling effect of sodium dodecylbenzene sulfonate (SDBS) and Na⁺ on the aggregation process of red soil colloid was studied using the dynamic light scattering method, and the mechanism of interactions between soil colloidal-sized particles and SDBS/Na⁺ was analyzed according to the pH and Zeta potential of suspension during the aggregation process. Results show that, 1under a given concentration of Na⁺, the soil colloidal suspension becomes more stable with increasing SDBS concentrations. For example, under 120 mmol·L^-1 Na⁺, as the concentrations of SDBS increase from 0 mmol·L^-1 to 10 mmol·L^-1, the effective diameters of aggregates decrease from 702 nm to 193 nm, and the total average aggregation rates of aggregates decrease from 28.6 nm·min^-1 to 3.36 nm·min^-1. 2Under a given concentration of SDBS, as the concentrations of Na⁺ increase, the Zeta potential of suspension sharply decreases, while the effective diameters and the total average aggregation rates of aggregates gradually increase. 3The absolute values of Zeta potential for suspensions without adding NaNO₃ solution increase from 47.6 mV to 62.2 mV as the SDBS concentrations increase, and the pH of the suspensions increase from 6.17 to 6.76, although these pH values are lower than that of initial soil colloidal suspension (6.89). Therefore, the adsorption of SDBS onto soil colloidal-sized particles, which is attributed to the hydrophobic effect and electrostatic effect, results in the increment of surface charge number, as well as the decrease in effective concentration of Na⁺ around colloidal-sized particles' surface (resulting from the steric hindrance of long hydrophobic chain of adsorbed SDBS and adsorption of Na⁺ by SDBS micelle). As a result, soil colloidal suspension becomes more stable and needs to absorb more Na⁺ to aggregate.