初始pH值对连续投加铝盐混凝去除小分子有机物的影响机制

以污水厂二级出水中小分子有机物强化混凝去除为目的,提出了连续投加混凝工艺(CDC),并以水杨酸为模型小分子有机物,研究了初始pH值对CDC工艺去除效果的影响以及铝离子与水杨酸的络合特性.结果表明,初始pH值6时CDC工艺的去除率最高,比常规混凝工艺提升了13.8%.但是初始pH值5和7时CDC工艺的提升效果较小.三维荧光结果表明,不同pH值下水杨酸和铝离子的络合特性不同.X射线光电子能谱和电喷雾质谱结果表明,pH值5时CDC工艺前期主要生成1:1络合物Al(OH)(C₇H₄O₃)(H₂O)₂.其络合作用强烈,稳定性较高,难以从水中去除.pH值6时前期主要生成中等聚合铝(如原位Al₁₃),其与水杨酸形成的络合物可以参与到铝离子的生长过程,最终生成表面崎岖形态松散的珊瑚礁状絮体,强化了水杨酸的去除.pH值7时,主要生成不定形氢氧化铝,通过絮体表面吸附小分子有机物.

Mechanisms of initial pH value affecting the removal of small molecular organics by coagulation with continuously added aluminum salts

To improve the removal efficiency of small molecular organics in the secondary effluent from the wastewater treatment plants, continuous dosing coagulation (CDC) was proposed. Salicylic acid (SA) was chosen as model small molecular organics to evaluate effects of initial pH value on the removal performance in the CDC process and the complexation characteristics of SA and aluminum. The CDC process with initial pH 6had the best removal efficiency that was 13.8% higher than that of conventional coagulation (CC). But the CDC process with initial pH 5 and 7 had limited promotion effect. According to the EEM results, the complexation characteristics of SA and aluminum varied with pH value. According to the XPS and ESI-MS results, 1:1 complexes Al(OH)(C₇H₄O₃)(H₂O)₂ formed in the early stage of CDC process with initial pH 5. The complexation reaction was strong so the stable complexes were difficult to remove. In the early stage of CDC process with initial pH 6, the medium polymer Al (such as in situ Al₁₃) formed and complexed with SA, which could polymerize with added aluminum and form coral reef-like flocs with rough surface and loose structure. As a result, the removal of SA was enhanced. The small molecular organics were removed by the adsorption of floc surface at initial pH 7 due to the formation of amorphous aluminum hydroxide.