Speciation matching mechanisms between orthophosphate and aluminum species during advanced P removal process.

Aluminum (Al) salts are widely used as coagulants to remove phosphorus (P) in water treatment. However, the relationship between P and Al species and the underlying coagulation mechanisms is rarely studied. Currently, water eutrophication is a serious issue, and therefore advanced P removal is extremely necessary. Herein, the orthophosphate removal behavior of Al coagulants with various species distributions was investigated. The results showed that AlCl3·6H2O (AC) had a more pronounced P removal efficiency than polyaluminum chloride (PACl). Medium (Alb or Al13) and high polymeric species (Alc) played a more significant role in removing P than monomeric species (Ala). During coagulation, adsorption onto flocs was the dominant P removal mechanism, which could be categorized as multilayer adsorption. Although the adsorption kinetics showed that physical adsorption best described the adsorption mechanism for AC and PACl, it is worth noting that chemical adsorption also occurred during P removal by AC because of the formation of the AlPO4 precipitate. This could be because of the strong complex adsorption between the in situ Al13 species and P. Based on the excellent P removal performance, we believe these findings will have a large potential for application in advanced P removal in water treatment.

Comparison of the effects of aluminum and iron(III) salts on ultrafiltration membrane biofouling in drinking water treatment.

Coagulation plays an important role in alleviating membrane fouling, and a noticeable problem is the development of microorganisms after long-time operation, which gradually secrete extracellular polymeric substances (EPS). To date, few studies have paid attention to the behavior of microorganisms in drinking water treatment with ultrafiltration (UF) membranes. Herein, the membrane biofouling was investigated with different aluminum and iron salts. We found that Al2(SO4)3·18H2O performed better in reducing membrane fouling due to the slower growth rate of microorganisms. In comparison to Al2(SO4)3·18H2O, more EPS were induced with Fe2(SO4)3·xH2O, both in the membrane tank and the sludge on the cake layer. We also found that bacteria were the major microorganisms, of which the concentration was much higher than those of fungi and archaea. Further analyses showed that Proteobacteria was dominant in bacterial communities, which caused severe membrane fouling by forming a biofilm, especially for Fe2(SO4)3·xH2O. Additionally, the abundances of Bacteroidetes and Verrucomicrobia were relatively higher in the presence of Al2(SO4)3·18H2O, resulting in less severe biofouling by effectively degrading the protein and polysaccharide in EPS. As a result, in terms of microorganism behaviors, Al-based salts should be given preference as coagulants during actual operations.

Modification of ultrafiltration membrane with nanoscale zerovalent iron layers for humic acid fouling reduction.

Nanoscale zerovalent iron (NZVI) was layered onto ultrafiltration (UF) membrane surface and tested for antifouling properties using humic acid (HA). Scanning electron microscopy showed that a relatively homogeneous layer was formed across the membrane surface by NZVI particles. Strong adhesion was observed between NZVI and UF membrane used. HA was significantly removed and membrane flux was increased in the presence of NZVI layer. Increased loadings of NZVI onto the membrane surface increased resistance to fouling while slightly reducing the clean water permeability of the membrane. However, the pore size of the layer formed by pristine NZVI was large, resulting in more chances of HA molecules getting to the membrane surface even blocking the membrane pores at the beginning. Membrane loaded with NZVI layer performed much better under acidic conditions. During NZVI synthesis, specific surface area of NZVI particle increased with increasing the ratio of ethanol (Vethanol/Vsolution), which also gradually decreased the average pore size of NZVI layer. As a result, the corresponding membrane flux steadily increased. Additionally, the results for permeate samples under different conditions showed that large molecular weight (MW, >30 kDa) and medium MW HA molecules (3-30 kDa) were removed much faster than those of small MW HA molecules (<3 kDa).