Coagulation of humic acid: The performance of preformed and non-preformed Al species

Abstract

Inorganic polymeric Al coagulants have been being widely used in water treatment due to their advantages of applicability within broad pH and temperature ranges, high capacity of charge neutralization as well as less alkalinity consumption. However, the removal of dissolved organic matter (DOM) by such kind of coagulants has not been well understood because of the complexity of natural organic matter and the diversity of such polymeric Al coagulants. In this paper, coagulants with different Al speciation characteristics were prepared to conduct coagulation of humic acid (HA). The humic acid used was peat origin and was characterized physicochemically. The results showed that coagulants with preformed Al species were less effective than conventional Al salt in removing humic acid with large molecular and hydrophobic properties. The flocs formed by preformed Al species were smaller than those formed by conventional Al salt. Decreasing pH could improve the coagulation performance of all coagulants. Coagulation of humic acid might not follow the same charge neutralization rules associated with coagulation of mineral colloids. In the presence of humic acid, Al 13 could be decomposed during coagulation process.

Preparation and characterisation of new-polyaluminum chloride-chitosan composite coagulant

“…An Al 13 species with the formula Al 13 O 14 ðOHÞ 24 ðH 2 OÞ 7þ 12 (abbreviated as Al 7þ 13 or often as Al 13 ) has been shown to be the dominant polymeric species in PACl Wang et al, 2008). These preformed polymerised Al species have the advantages of being more effective under a variety of water quality conditions compared to monomeric Al 3þ species in alum, especially at lower temperatures and a broader pH ranges (Shi et al, 2007). Although the prehydrolysed polymer coagulants have been reported to have advantages over conventional coagulants, a number of studies found that in certain conditions the coagulation efficiency of preformed Al species (Al 13 ) is not better than that of the conventional Al salt (Hu et al, 2006;Shi et al, 2007).…”

“…These preformed polymerised Al species have the advantages of being more effective under a variety of water quality conditions compared to monomeric Al 3þ species in alum, especially at lower temperatures and a broader pH ranges (Shi et al, 2007). Although the prehydrolysed polymer coagulants have been reported to have advantages over conventional coagulants, a number of studies found that in certain conditions the coagulation efficiency of preformed Al species (Al 13 ) is not better than that of the conventional Al salt (Hu et al, 2006;Shi et al, 2007). More recently, research interest has been directed towards improving the coagulation and flocculation efficiency of pre-polymerised metal coagulants by incorporating polyelectrolytes or silica into their formulation (Ahmad et al, 2008;Zouboulis and Tzoupanos, 2009).…”

“…4. At pH 6, NOM is known to have a negative surface charge due to dissociation of carboxyl functional groups (Shi et al, 2007;Liu et al, 2009). PACl and chitosan, in contrast, have a positive surface charge due to the presence of monomeric and polymeric aluminium species as well as eNH 2 functional groups on the PACl and chitosan molecules, respectively.…”

Evaluation of enhanced coagulation coupled with magnetic ion exchange (MIEX) in natural organic matter and sulfamethoxazole removals: The role of Al-based coagulant characteristic

“…Besides, flocs formed by Al c were found to be larger than those by other species, which is favor of the settling efficiency. Comparatively, most researchers claimed that prehydrolyzed Al b with high positive charge shows applicability within broad pH and could destabilize particles more efficiently [8,9]. It has been proven that hydrophobic and high molar mass fractions of NOMs can be efficiently removed by enhanced coagulation (EC) [10].…”

Coagulation of humic acid by PACl with high content of Al13: The role of aluminum speciation

“…In addition to the charge neutralization precipitates formed during the coagulation process, the adsorption on amorphous metal hydroxide precipitates contributes the HA removal [20][21][22]. Generally, charge neutralization occurs at low pH values only soluble species are present, while adsorption means the appearance of bulk hydroxide precipitation at high pH values or at high coagulant doses.…”