Why is the coagulation effect of polymeric ferric sulfate sometimes superior to that of polymeric aluminum chloride

When conducting coagulation and sedimentation experiments, many people subconsciously prefer to use liquid polymeric ferric sulfate (PFS) because it is easy to achieve results, produces large flocs, precipitates quickly, and yields a clear supernatant. The disadvantage is that adjusting the pH value requires the consumption of liquid alkali, which increases material costs. Overdosing can also increase the chromaticity of the effluent. Although there is no such problem with polyaluminium chloride (PAC), in comparative experiments with the same water sample, PAC coagulation does yield a slightly worse sensory effect. From 100mg/L to 300mg/L, there was no significant improvement in the sensory properties of the supernatant with increasing dosage. This indicates that there are significant differences in the coagulation properties of polymeric ferric sulfate (PFS) and polymeric aluminum chloride (PAC). (1) Charge density and electric neutralization effect The core first step of coagulation is the electric neutralization effect. The positive charges of the hydrolysis products of coagulants interact with negatively charged colloids (such as clay particles and organic matter) in water, reducing the zeta potential of the colloids and breaking their stability. After losing three electrons, Fe³⁺ is formed, with five empty orbitals remaining in its 3d orbitals, exhibiting stronger coordination ability and capable of forming polynuclear complexes with water molecules and hydroxyl groups (OH⁻). These products have high polymerization degrees and concentrated positive charge distribution. On the other hand, Al³⁺ has fully empty 3p orbitals, resulting in weaker coordination ability, mainly forming low-polymerization-degree hydrolysis products. Moreover, high-polymerization products such as Al₁₃ can only exist stably within a narrow pH range. Especially under acidic conditions, the hydrolyzed products of PFS carry more positive charges, have a stronger ability to compress the double layer, and can more effectively neutralize the negative charges of colloids in water, reducing their zeta potential and promoting colloidal coagulation. However, the low-polymerization products of PAC have insufficient charge supply under acidic conditions, weakening the electric neutralization effect. (2) Stability of hydrolyzed products The long-lasting effectiveness of coagulants relies on the structural stability of their hydrolysis products in complex water quality. The bond energy of Fe-O bond is approximately 400 kJ/mol, which is higher than that of Al-O bond (318 kJ/mol). Stronger chemical bonds endow the hydrolysis products of PFS with superior structural stability, and the effective charge can be effectively maintained within the pH range of 4-11. However, the hydrolysis products of PAC have a higher charge density under neutral to weakly alkaline conditions. When pH is less than 6 or greater than 8, they are susceptible to pH fluctuations, leading to bond breakage, structural collapse of the products, and reduced coagulation efficiency. (3) Rolling and sweeping capability The second step of coagulation, known as the sweeping action, essentially involves the physical capture of minute colloids by the hydrolytic products generated by the coagulant. Its effectiveness depends on physical characteristics such as the density and specific surface area of the products. The crystal structure of Fe(OH)₃ generated by PFS hydrolysis is hexagonal, with a sediment density of approximately 3.4g/cm³. The crystals grow in a “branch-like” manner, forming flocs with a porous network structure and a large specific surface area. This fishnet-like structure can more efficiently encapsulate tiny colloids (such as fine sediment and humic acid molecules with a particle size <1μm). the al(oh)₃ generated by pac hydrolysis belongs to trigonal crystal system, with a density of approximately 2.4g/cm³. grows in "layered" manner, resulting loose floc structure relatively small specific surface area. it is difficult fully capture tiny particles, and presence some "slip-throughs" can lead high turbidity effluent. The main products of flocculant manufacturer Xinhuan Water Treatment are: White polyaluminium chloride (food grade), drinking grade polyaluminium chloride, industrial grade polyaluminium chloride, anionic polyacrylamide, cationic polyacrylamide, nonionic polyacrylamide, petroleum recycling polyacrylamide, oil recovery polyacrylamide, polyacrylamide, polymer flocculant polymer flocculation coagulant, polymeric ferrous sulphate, polymerized iron and aluminium chloride, and other water treatment chemicals. Welcome customers to write to us to negotiate business! Tel/WhatsApp：86 19139972558 Email：Sunny@xhwtm.com Contact:Sunny