Study on the hydrolysis/precipitation behavior of Keggin Al13 and Al30 polymers in polyaluminum solutions

Abstract

a b s t r a c tThe hydrolysis/precipitation behaviors of Al 3þ , Al 13 and Al 30 under conditions typical for flocculation in water treatment were investigated by studying the particulates' size development, charge characteristics, chemical species and speciation transformation of coagulant hydrolysis precipitates. The optimal pH conditions for hydrolysis precipitates formation for AlCl 3 , PAC Al13 and PAC Al30 were 6.5-7.5, 8.5-9.5, and 7.5-9.5, respectively. The precipitates' formation rate increased with the increase in dosage, and the relative rates were AlCl 3 [ PAC Al30 > PAC Al13 . The precipitates' size increased when the dosage increased from 50 mM to 200 mM, but it decreased when the dosage increased to 800 mM. The Zeta potential of coagulant hydrolysis precipitates decreased with the increase in pH for the three coagulants. The isoelectric points of the freshly formed precipitates for AlCl 3 , PAC Al13 and PAC Al30 were 7.3, 9.6 and 9.2, respectively. The Zeta potentials of AlCl 3 hydrolysis precipitates were lower than those of PAC Al13 and PAC Al30 when pH > 5.0. The Zeta potential of PAC Al30 hydrolysis precipitates was higher than that of PAC Al13 at the acidic side, but lower at the alkaline side. The dosage had no obvious effect on the Zeta potential of hydrolysis precipitates under fixed pH conditions. The increase in Zeta potential with the increase in dosage under uncontrolled pH conditions was due to the pH depression caused by coagulant addition. Al-Ferron research indicated that the hydrolysis precipitates of AlCl 3 were composed of amorphous Al(OH) 3 precipitates, but those of PAC Al13 and PAC Al30 were composed of aggregates of Al 13 and Al 30 , respectively. Al 3þ was the most un-stable species in coagulants, and its hydrolysis was remarkably influenced by solution pH. Al 13 and Al 30 species were very stable, and solution pH and aging had little effect on the chemical species of their hydrolysis products. The research method involving coagulant hydrolysis precipitates based on Al-Ferron reaction kinetics was studied in detail. The Al species classification based on complex reaction kinetic of hydrolysis precipitates and Ferron reagent was different from that measured in a conventional coagulant assay using the Al-Ferron method. The chemical composition of Al a , Al b and Al c depended on coagulant and solution pH. The Al b measured in the current case was different from Keggin Al 13 , and the high Al b content in the AlCl 3 hydrolysis precipitates could not used as testimony that most of the Al 3þ was converted to highly charged Al 13 species during AlCl 3 coagulation.

Coagulation behavior of polyaluminum chloride: Effects of pH and coagulant dosage

“…This indicates that the optimal mechanism at pH 6.0 for PACl is charge neutralization and the optimal pH for coagulation with PACl is above 7.0. These results could be related to the formation of PACl hydrolysis precipitates under alkaline conditions [40]. At pH 7.0-10.0, sweep coagulation and charge neutralization have similar RT, but the former occurs over a wider range of dosages than the latter.…”

Influence of coagulation mechanisms on the residual aluminum – The roles of coagulant species and MW of organic matter

“…These coagulants are Al 2 (SO 4 ) 3 , Al 13 and Al 30 , which represented Al a -coagulant, Al b -coagulant and Al c -coagulant, respectively. Except Al 2 (SO 4 ) 3 (commercial), the other two kind of coagulants were prepared in the laboratory.…”

“…Al 13 and Al 2 (SO 4 ) 3 only presented one peak at 63 ppm and 0 ppm, respectively, which indicated the existence of [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ and Al monomer [24]. Compared to the other coagulants, Al 30 [20]. The structure of Al 30 is complex and includes two ␦-Al 13 and four hexacoordinated Al-oxygen octahedrons.…”

“…Among them, only the two four-coordinated Al s of ␦-Al 13 , which were Cc symmetric could show resonance peak at 70 ppm. As a result, the concentration of Al 30 in the result of NMR needs to multiply by fifty [25]. Therefore, even though it seemed that the peak at 0 ppm in Al 30 had stronger resonance intensity than the peak at 70 ppm, it is reasonable to confirm Al 30 is the main species in the laboratory that prepared Al 30 coagulant.…”

“…The chemical species of hydrolyzed Al(III) can be divided into three types by different reaction rates: monomeric species (Al a ), medium polymer species (Al b ), and species of sol or gel (Al c ) [26]. As shown in Table 1, Al a was the main species in Al 2 (SO 4 ) 3 , while Al 13 and Al 30 represented Al b and Al c , respectively. The results were positively correlated with those obtained by NMR.…”