柠檬酸对不同混凝剂混凝效果的影响

通过对不同混凝剂(Al2(SO4)3、Fe2(SO4)3、PACl25)混凝过程中絮体粒径大小的测定,研究了在不同pH条件下柠檬酸浓度对3种混凝剂混凝效果的影响.结果发现,随着柠檬酸浓度的增加,3种混凝剂混凝过程中生成絮体的速率和粒径大小都在逐渐降低,但3种混凝剂受到抑制的程度不同:对于硫酸铝,当n(柠檬酸)/n(Al) =0.1时,混凝20 min内没有絮体生成;对于硫酸铁,仅当n(柠檬酸)/n(Fe)=0.05时,絮体就不再生成;而当n(柠檬酸)/n(Al) =0.5时,PACl25絮体仍能很快地形成.不同的pH值下柠檬酸对混凝效果的抑制程度也不同:对于硫酸铝,pH =6时生成絮体最大,pH =8时没有絮体生成;对于硫酸铁,pH =5时产生絮体的情况和pH =6时相似,而pH =7时絮体消失;而对于PACl25,絮体大小随着pH值的升高而增大(pH在7～9之间).另外,通过对Zeta电位的测定可知,随着柠檬酸浓度的增加,3种混凝剂的Zeta电位都在降低,但降低的速率不同:硫酸铁＞硫酸铝＞ PACl25,这与絮体粒径大小的变化规律一致.

Effect of citric acid on coagulation process with different coagulants

The effect of citric acid on different coagulant processes with different coagulants was explored here, including aluminium sulphate, ferric sulphate and polyaluminum chloride (PACl₂₅). Results showed that the growth rate and size of flocs both decreased with an increasing concentration of citric acid for three coagulants, but some differences existed. There was no flocs formed in 20 minutes at a citric acid/Al(Fe) molar ratio of 0.1(alum) and 0.05 (ferric sulphate), but for polyaluminum chloride (PACl₂₅), flocs were still able to form quickly at a citric acid/Al molar ratio of 0.5. In addition, citric acid inhibited coagulation efficiency at different pH values. Alum flocs were the largest at pH 6, and there was no floc formed at pH 8. The formation of flocs were similar at pH 5 and 6 for Fe₂(SO₄)₃, but the flocs disappeared at pH 7. On the contrary, the size of flocs increased as pH increased from 7 to 9 for PACl₂₅. Zeta potential results showed that with an increasing concentration of citric acid, the zeta potential of flocs with different species of coagulants decreased as Fe₂(SO₄)₃ > alum > PACl₂₅. These results supported the variation of flocs formed by different species of coagulants influenced by citric acid.