Elsevier

Science of The Total Environment

Volume 642, 15 November 2018, Pages 1311-1319
Science of The Total Environment

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

https://doi.org/10.1016/j.scitotenv.2018.06.171Get rights and content

Highlights

  • AlCl3·6H2O performed better than PACl with regard to the advanced P removal.

  • Species of Alb and Alc played a more important role in removing P than Ala species.

  • In situ Alb species showed better behavior than the prepared Alb species.

  • Floc adsorption was the dominant P removal mechanism.

Abstract

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.

Introduction

With rapid economic development in recent years, large amounts of phosphorus (P) have entered reservoirs, lakes, or rivers, resulting in an urgent need for effective P removal (Wang et al., 2005; Guan et al., 2007; Karageorgiou et al., 2007). To avoid various environmental problems, the effluent standard for P in wastewater in many countries has become much stricter. The European Union has now set the minimum effluent standard for total P in wastewater treatment at 1 mg/L, while the requirement is even stricter in other countries (0.5 mg/L) because of the current severity of environmental conditions (Karageorgiou et al., 2007; Parsons and Smith, 2008; Hussain et al., 2011).

Phosphate salts in water always occur in dissolved or particulate states, which can be classified as orthophosphates (e.g., PO43−, HPO42−), condensed phosphates (e.g., pyrophosphates, metaphosphates), and organically bound phosphates (e.g., malathion, parathion) (Sommariva et al., 1997; Georgantas and Grigoropoulou, 2007). According to the characteristics of P, physical/chemical and biological methods have been investigated to effectively remove P. For the biological methods, the liquid phase containing phosphate salts must be transferred to the sludge phase. Moreover, the removal efficiency is always lower than 30% (Sommariva et al., 1997). To meet the standard requirements, the remaining phosphate salts should be removed by other technologies (Sommariva et al., 1997; Bektaş et al., 2004). For physical/chemical methods, various calcium, aluminum, and iron salts are used, with no secondary pollution occurring during sludge disposal (Fytianos et al., 1998; Özacar and Şengil, 2003; Kordlaghari and Rowell, 2006; Lacasa et al., 2011).

Coagulation is the most commonly used physical/chemical method due to its simplicity and ease of operation (Henneberry et al., 2011; Sheng et al., 2016; Wang et al., 2018). As a result, many studies have focused on the P removal mechanisms during coagulation (Boisvert et al., 1997; İrdemez et al., 2006a; Zhou et al., 2008; Tran et al., 2012; Nguyen et al., 2016), with much attention being paid to the influential factors such as the characteristics of aluminum or iron salts and their corresponding removal efficiencies (Morse et al., 1998; Tanada et al., 2003; Xie et al., 2005; Zhou et al., 2008; Kawasaki et al., 2010). However, little attention has been paid to the relationship between P and aluminum species. It is well known that various kinds of Al species exist in water, such as Al3+, Al2(OH)24+, Al13O4(OH)247+, and AlO2 (Georgantas and Grigoropoulou, 2007; Zhao et al., 2009; Feng et al., 2015). Compared to other aluminum species, the Al13 species (Al13O4(OH)247+), which can be formed in situ through hydrolysis or prepared by electrolysis, is considered to be the most active during coagulation because of its high positive charge and large specific surface area (Parker and Bertsch, 1992; Teien et al., 2004; Tchamango et al., 2010; Ma et al., 2012; Feng et al., 2015). Using the ferron colorimetric method, the aluminum species can be characterized as Ala (monomeric) species, Alb (medium polymeric) species, and Alc (high polymeric) species. These correspond to the Alms (monomeric and small polymeric aluminum species), Alml (medium and large polymeric aluminum species), and Alu (undetected or solid phase hydrolysis products), respectively, measured using the 27Al nuclear magnetic resonance (NMR) method (Vilge et al., 1999).

Previous studies have demonstrated that the removal mechanism for pollutants was related to the types of aluminum species due to the differences in their characteristics. For organic matters, it has been reported that polysaccharides and cellulosic molecules are easily combined with monomeric aluminum species (Vilge et al., 1999; Masion et al., 2000). Carboxylic groups and phenolic moieties are easily combined with preformed Alb (mainly Al13) species (Kazpard et al., 2006). Other substances with aliphatic moieties are easily removed/adsorbed by hydrolyzed flocs (Zhao et al., 2009). For inorganic matter, it has been demonstrated that the removal efficiency for arsenic(V) is positively related to the concentration of Al13 species (Hu et al., 2012a), while freshly prepared aluminum hydroxide plays an important role in fluoride, copper(II), and chromium(VI) removal (Adhoum et al., 2004; Liu et al., 2011). For actual water treatment plants, understanding the relationship between pollutants and Al species is not only beneficial for improving pollutant removal, but also helpful in reducing the operating cost of water treatment.

It has been reported that the proportion of orthophosphate in domestic wastewater is significantly higher compared to that of condensed phosphates and organically bound phosphates (Sommariva et al., 1997). However, for orthophosphate removal, most studies related to Al-based coagulants have focused on the removal efficiency (İrdemez et al., 2006b; Tian et al., 2009; Guaya et al., 2015), influential factors (Omoike and Vanloon, 1999; Xie et al., 2005; Kawasaki et al., 2010), and adsorption behavior onto aluminum hydroxide (Boisvert et al., 1997; Tanada et al., 2003; Guan et al., 2007). The relationship between aluminum species and P has received only little attention. Additionally, a noticeable problem is that the corresponding P concentration is high, typically higher than 10 mg/L. It is difficult to meet the strict P effluent standards during actual operations, thus necessitating advanced P removal. To effectively remove P, the relationship between P and aluminum species was investigated in this study in the presence of AlCl3·6H2O (AC) and polyaluminum chloride (PACl) under various pH conditions. The objective is to understand: (1) the removal behavior of orthophosphate at low concentrations with various Al-based coagulants, (2) the contribution of various aluminum species to orthophosphate removal, and (3) the micro-scale interface mechanism between orthophosphate and Al species during the coagulation process.

Section snippets

Characteristics of synthesized water sample

To simulate natural surface water, the feed water was prepared using tap water (Beijing, China) diluted with an identical volume of deionized (DI) water (specific characteristics are shown in Table 1). For the experiments, all chemical regents (Sinopharm Chemical Reagent Co., Ltd., Beijing) used were analytical grade except when specified. The total concentration of P in KH2PO4 was 1 mg/L.

Characterization of aluminum speciation

AlCl3·6H2O was used as the conventional coagulant, while PACl was prepared using an electrolysis process.

P removal by aluminum coagulants with various Al speciation

Previous studies showed that the potential formation of precipitation was high between P and Ca2+/Mg2+ (Kordlaghari and Rowell, 2006; Chen et al., 2016). Thus, the concentration of TP and Ca2+/Mg2+ before and after the reaction was investigated. As seen from Table S1, minimal variation occurred in the concentrations of TP, Ca2+, and Mg2+ in the influent and the effluent, indicating that little precipitation was produced. The potential reasons were the low concentration of Ca2+ and Mg2+ and the

Conclusions

Although the distribution of aluminum species varies greatly as a function of pH, little attention has been focused on the relationship between aluminum species and P, especially during advanced P removal processes. Here, three kinds of aluminum salts with different aluminum speciation were investigated in the presence of orthophosphate at low concentrations under various pH conditions. We found that the P removal efficiency was high after coagulation with Al-based salts. In comparison with the

Acknowledgements

This study was supported by the National Natural Science Foundation of China (51378490), National Natural Science Foundation for Young Scientists of China (51608514), and a special fund from the Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (Project No. 17Z03KLDWST).

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