Competitive immobilization of Pb in an aqueous ternary-metals system by soluble phosphates with varying pH
Graphical abstract
Introduction
The use of phosphates to immobilize Pb in wastewater and contaminated soils has been well documented attributed to the formation of highly insoluble Pb-phosphate minerals (e.g., pyromorphite minerals) (Suzuki et al., 1984, Cao et al., 2004a, Miretzky and Fernandez-Cirelli, 2008, Zhang et al., 2015). In many instances, wastewaters and soils are subjected to pollution by combinations of multiple metals including Pb, Zn, Cu, Cd, or other metals due to ore mining, smelting and etc. (Fang et al., 2012, Sdiri et al., 2012). Some previous studies have attempted to stabilize single Zn or Cd in soil by phosphorus amendments. The findings of several studies indicated that phosphate minerals of Zn and Cd may have been formed, resulting in reduced extractability and toxicity (Brown et al., 2005, Chen et al., 2007, Kumpiene et al., 2008, Williams et al., 2011), and there were some evidences that phosphate amendments significantly lowered the soil pH and affected Zn speciation and enhanced Zn mobility (Baker et al., 2012). For multiple metals pollution, some investigations have shown that Zn and Cd could interact with phosphate amendments simultaneously, which led to the reduction of extractability and toxicity (Chen et al., 2007, Williams et al., 2011). However, other studies have shown only a slightly reduced Zn solution concentration after application of phosphoric acid, while Pb, Cd and Cu concentrations were simultaneously decreased five- to tenfold (Impellitteri, 2005). Accordingly, the mechanisms of phosphate-based immobilization of Pb, Zn and Cd are still not fully understood.
In the multi-metal system, metal ion composition strongly affects metal reaction with phosphates (Cao et al., 2004b), while competitive reactions might affect the effectiveness of Pb immobilization by phosphates. To our knowledge, existing research regarding the process of competitive reaction of metals with phosphate is scarce, and these studies have only focused on phosphate rock with a lower solubility. Cao et al. found that competitive sorption occurred with an interfering effect in the order of Pb > Cu > Zn in a study of a reaction between phosphate rock and metals (Cao et al., 2004b). Corami et al. showed that, in a multi-metal (Cd + Pb + Zn + Cu) aqueous solutions, Cd sorption by hydroxyapatite decreased by 63%–83% for competitive metals relative to single-metal aqueous solutions (Corami et al., 2008).
Logically, the reaction to form heavy metal-phosphate minerals is rate limited by the presence of metal ion and phosphate anion in the soil solution (Scheckel et al., 2013). The heavy metals ions in the soil solution are most reactive, chemically and biologically. Thus it is critical to understand the mobility, transport, and fate of metals in soils (Carrillo et al., 2006). Phosphate sources could be divided into three major categories: readily soluble phosphates, such as phosphoric acid; moderately soluble phosphates; such as mono-, di- and tricalcium/potassium/sodium phosphate; and less soluble phosphates, such as calcium hydroxyapatite and phosphate rock (Shigeru et al., 2003, Miretzky and Fernandez-Cirelli, 2008). Different phosphate sources may impact the effectiveness of Pb immobilization based on their solubility, and soluble phosphates facilitate Pb stabilization (Miretzky and Fernandez-Cirelli, 2008, Scheckel et al., 2013). Moreover, phosphate sources showed different effectiveness during Pb reactions with significant pH variation (Chrysochoou et al., 2007). Although the solubility and pH of phosphates have been shown to dictate the rate and extent of Pb-phosphate minerals formation, studies of the effects of phosphates with varying pH on immobilization are rarely studied. In this study, soluble KH2PO4, K2HPO4, and K3PO4 were selected to comprehensively evaluate the effectiveness and stability of Pb immobilization in the presence of other metals (e.g., Zn, Cd) over a wide pH range.
Since the reaction of heavy metal-phosphate minerals formation mainly occurred in soil solution, competitive reaction of metals with phosphate was designed in the aqueous solution to better understand the immobilization process in this study. The objectives of this study were: (1) to evaluate the effectiveness of Pb, Zn, and Cd immobilization in single-metal systems by soluble phosphates with varying pH (KH2PO4, K2HPO4, and K3PO4); (2) to examine the competitive immobilization of Pb in a ternary-metals (Pb + Zn + Cd) system based on a series of batch experiments; (3) to determine the stability of Pb after immobilization by phosphates with varying pH through solubility experiments; (4) to characterize differences of structures and compositions of Pb minerals formed by phosphates with varying pH through SEM and XRD.
Section snippets
Reagents and materials
The phosphates used in this study were bought from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All reagents were of analytical grade, and deionized water was used for all the experiments. Individual aqueous solutions of Pb, Zn, and Cd were prepared at concentrations of 10 mM from their nitrate salts. The experiments were run in triplicate.
Immobilization reaction in single-metal and ternary-metals system
Three treatments were designed to evaluate KH2PO4, K2HPO4, and K3PO4. A series of phosphates containing different concentrations (0–40 mM) and
pH-dependent immobilization rates by phosphates in a single-metal system
For a single-metal system, the immobilization rates of Pb, Zn, and Cd by different phosphates are shown in Fig. 1. Immobilization reaction with phosphate for Pb retained slight differences with the varying pH (Fig. 1). KH2PO4, K2HPO4, and K3PO4 were applied to solution at a P:Pb molar ratio of 2:3, which satisfied the lead phosphate stoichiometry, and the immobilization rates were 57.1%, 92.5%, and 99.9%, respectively.
The reaction behaviors of Zn and Cd were significantly different from Pb. The
Conclusions
Phosphates are able to effectively immobilize Pb from aqueous solutions in both single- and ternary-metals (Pb + Zn + Cd) system. In the ternary-metals system, competitive reaction occurred among the three metals. The immobilization rate of Pb was higher than Zn and Cd and exhibited intense competitiveness and specificity in ternary-metals system. The crystallinity and mineral composition of Pb immobilized by phosphates with varying pH induced different Pb stability in ternary-metals system. To
Acknowledgments
This study was financially supported by the National High Technology Research and Development Program (“863” Program) of China (No. 2013AA06A206) and the National Natural Science Foundation of China (Grant No. 41571311).
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