Immobilization of Zn, Cu, and Pb in contaminated soils using phosphate rock and phosphoric acid

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Abstract

Considerable research has been done on P-induced Pb immobilization in Pb-contaminated soils. However, application of P to soils contaminated with multiple heavy metals is limited. The present study examined effectiveness of phosphoric acid (PA) and/or phosphate rock (PR) in immobilizing Pb, Cu, and Zn in two contaminated soils. The effectiveness was evaluated using water extraction, plant uptake, and a simple bioaccessibility extraction test (SBET) mimicking metal uptake in the acidic environment of human stomach. The possible mechanisms for metal immobilization were elucidated using X-ray diffraction, scanning electron microscopy, and chemical speciation program Visual MINTEQ. Compared to the control, all P amendments significantly reduced Pb water solubility, phytoavailability, and bioaccessibility by 72–100%, 15–86%, and 28–92%, respectively. The Pb immobilization was probably attributed to the formation of insoluble Pb phosphate minerals. Phosphorus significantly reduced Cu and Zn water solubility by 31–80% and 40–69%, respectively, presumably due to their sorption on minerals (e.g., calcite and phosphate phases) following CaO addition. However, P had little effect on the Cu and Zn phytoavailability; while the acid extractability of Cu and Zn induced by SBET (pH 2) were even elevated by up to 48% and 40%, respectively, in the H3PO4 treatments (PA and PR + PA). Our results indicate that phosphate was effective in reducing Pb availability in terms of water solubility, bioaccessibility, and phytoavailability. Caution should be exercised when H3PO4 was amended to the soil co-contaminated with Cu and Zn since the acidic condition of SBET increased Cu and Zn bioaccessibility though their water solubility was reduced.

Introduction

Soil contamination with heavy metals is of great concern. The primary sources of Pb contamination include industrial activities such as mining, smelting of metals and the use of Pb-containing products such as paint, gasoline, and pesticides [1]. Recently, the use of Pb bullets/shot as ammunition at shooting ranges is under increasing scrutiny as a potentially significant source of Pb pollution [2]. Battery recycling sites are often elevated with Pb as well as other metals (Cd, Cu, As, Sb, and Se) [3]. Mining and smelting of Cu and Zn metal ores are important sources of Cu and Zn environmental degradation to soil and water sources [4]. Although Cu and Zn are not a human health concern, their phytotoxic levels can result in soil erosion by wind and water, thereby increasing human exposure to other metal contaminants (Pb and Cd) [5].

Human exposure to contaminated soils includes leaching of metals from the soil into water and consumption of edible plants grown in the contaminated soil [5], [6]. In addition, soil and dust ingestion by humans, especially children, is of health concern [7]. Therefore, implementing soil remediation practices to reduce metal availability in soils is necessary to protect human health. In situ chemical immobilization is a cost-effective remediation approach for the reduction of metal mobility and bioavailability in contaminated soils. Amendments added to the soil immobilize a contaminant and reduce leachable concentrations to an acceptable level [5]. Phosphate has been shown to be effective in immobilizing Pb in contaminated soils via formation of stable Pb phosphate minerals [8], [9]. Lead phosphates, in particular pyromorphite [Pb5(PO4)3(F, Cl, OH)], are the most insoluble form of Pb minerals in soils under a wide range of environmental conditions [10], [11]. The suggested P/Pb molar ratios for effective remediation of Pb-contaminated soils range from 3/5 to 4/1 depending on the presence of other metals (e.g., Cd and Cu) that may compete with Pb for dissolved P [1], [5].

Previous studies using P amendments mainly focused on Pb immobilization in contaminated soils [1], [8], [12], [13], [14]. However, there is only limited work addressing the effects of P application on immobilization of other metals (e.g., Cu, Zn, and Cd) [5], [15], [16], [17]. A single amendment may not be suitable for multiple metals and the treatment effectiveness depends on assessment methods [18], [19], [20]. For example, addition of P to a soil contaminated with Pb, Zn, and Cd reduced the bioavailable Pb, but increased plant Cd concentrations [19]. Phosphate rock was effective in reduced Pb in physiologically based extraction test (PBET) but failed to prevent Pb phytotoxicity and Pb plant uptake on all soils tested [21].

The purpose of this study was to evaluate the effectiveness of phosphoric acid and/or phosphate rock in immobilizing Pb, Cu, and Zn in two contaminated soils. The specific objectives were to (i) determine P-induced metal transformation in soils using X-ray diffraction, scanning electron microscopy and chemical speciation program Visual MINTEQ [22] and (ii) estimate the availability of Pb, Cu, and Zn after P amendment using water extraction, plant uptake, and a simple bioaccessibility extraction test (SBET) [23].

Section snippets

Sampling and characterization

The two soils used in this study were collected from the upper 20 cm of an abandoned battery recycling (BR) site and a berm of a shooting range (SR) in north central Florida, USA. The BR site was used for battery recycling and as a salvage yard for the discharge of urban wastes from 1940s to 1980s [17], while the SR site has been in operation since the early 1990s [24]. After being air-dried, the soil samples were passed through a 2-mm sieve. Physical and chemical properties of the two soils are

Soil characterization

Selected physical and chemical properties of the two soils are presented in Table 1. Both soils were very sandy (>72% sand) with pH of 6.3–7.0. The BR soil contained a greater level of organic matter (∼7%) than the SR soil (∼1%). Higher organic matter in BR soil mainly originated from the urban waste. Lime addition to neutralize acidity from lead battery probably resulted in the increase of soil pH [13]. Both soils were mainly contaminated with Pb, with concentrations being ∼7500 mg kg−1 in the

Conclusions

Without exception, all amendments reduced Pb availability in this experiment, showing a significant reduction of water soluble, phytoavailable, and bioaccessible Pb, presumably due to formation of insoluble Pb phosphate (e.g., pyromorphite-like mineral).

Phosphate amendments also lowered water solubility of Cu and Zn, probably due to their sorption onto soil minerals, e.g., calcite, phosphate minerals following CaO addition. However, P had little effect on the phytoavailability of Cu and Zn,

Acknowledgments

This research was supported in part by the Florida Institute of Phosphate Research and the Ministry of Science and Technology of Peoples’s Republic of China (No. 2006DFA21280). The authors would like to thank Mr. Thomas Luongo for his assistance in chemical analysis.

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