Formation of Al30 aggregates and its correlation to the coagulation effect

Al₃₀ is the polycation with the highest degree of polymerization and surface charge in the currently known structural aluminum species. It shows excellent coagulation performance in water treatment process, and has the characteristics of wide application range of pH and dosage. pH value is one of the most important factors affecting the aggregation and coagulation process of Al₃₀, but the influence of Al₃₀ aggregation reaction on its coagulation effect is still unclear. Therefore, this article reports the deprotonation and aggregation reaction of Al₃₀ by adjusting the basicity (B) of the solution, particularly to further understand the coagulation mechanism of Al₃₀ under different conditions. The results showed that in the base titration process, when B < 2.86 in 0.01 M Al₃₀ solution as Al_T (the concentration of total Al), deprotonation and preliminary aggregation mainly occurred; when B > 2.86, the size of Al₃₀ aggregates (Al₃₀agg) increased rapidly, forming gels and gradually transforming into Al(OH)₃. In this process, in addition to the reduction of electrostatic repulsion induced by Al₃₀ deprotonation, the oligomers generated by the partial dissociation of Al₃₀ also play the role of bridging-connection. Under the experimental titration conditions, the Al₃₀agg always maintained a positive zeta potential. In addition, Al₃₀ can deprotonate and aggregate at lower pH, which is an important reason for its unique coagulation characteristics. The larger structure size of Al₃₀ also made it easy to form branched aggregates, so that it can play an effective role in a wider dosage range without destabilization of colloids. This study gives an insight in the advancement of coagulants and promotes the industrial application and commercialization of functional coagulants based on polyaluminum.

Humic Acid Removal from Water with PAC-Al30: Effect of Calcium and Kaolin and the Action Mechanisms

Polyaluminum chloride with a dominant species of Al30 (PAC-Al30) was prepared in laboratory and used for humic acid (HA) removal from water. The action properties and mechanisms of PAC-Al30, HA, calcium, and kaolin were tested and discussed. The results showed that the existence of calcium or kaolin contributed to the HA removal when the PAC-Al30 dosage was deficient and had no obvious effect when the amount of PAC-Al30 was sufficient. When the PAC-Al30 dosage was 0.01 and 0.02 mmol/L, the HA removal rate was increased by 66.59 and 42.20%, respectively, with a calcium concentration of 2.0 mmol/L, or increased by 53.31 and 40.92%, respectively, with the kaolin particle concentration of 150 mg/L. Calcium could compress the double electrical layers or complex with HA to neutralize a part of the surface negative charge of HA, but could not make the water system reach its isoelectric point. The mechanisms of calcium and kaolin's promoting coagulation effect were adsorption neutralization and collision aggregation respectively, but these actions were much weaker than that of PAC-Al30 with HA. The adsorption neutralization capacity of PAC-Al30 was calculated to be nearly 60 times than that of calcium, and the higher γ value of calcium modified by the Sips equation may indicate that the adsorption or neutralization sites of calcium on HA were pickier than PAC-Al30.

High-poly-aluminum chloride sulfate coagulants and their coagulation performances for removal of humic acid

High-poly-aluminum chloride sulfate (HPACS) coagulants with different [SO₄²⁻]/[Al³⁺] molar ratio (S) were prepared and proved to have high coagulation efficiency for the removal of humic acid and strong stability for storage and application. The results showed that the higher the SO₄²⁻ addition, the bigger the aluminum polymerization particles and the more the polymerization Al_c existed in the prepared HPACS coagulants. The HPACS exhibited higher coagulation efficiency, a better aging stability and stronger resistance to the change of pH and Ca²⁺ concentration of raw water than the polyaluminum chloride (PAC) and poly-aluminum chloride sulfate (PACS) reported before. The Sips adsorption neutralization model was established to illustrate the relationship between coagulant dosage and zeta potential of the water system. The adsorption neutralization capacity was proved to be HPACS (S = 0) > HPACS (S = 0.02) > HPACS (S = 0.06) > HPACS (S = 0.10), which was not completely consistent with the coagulation effect of HPACS with different S values and indicated that in addition to adsorption neutralization, actions like bridge-aggregation, precipitation, and sweep-flocculation also played an important role during HPACS coagulation. Moreover, the negative Gibbs free energy indicated that the coagulant adsorption neutralization reaction was a spontaneous process.

Effect of aging time of HPACS on HA removal (with a minimum absolute deviation of 5%).