Evaluation of hydroxyapatite as a metal immobilizing soil additive for the remediation of polluted soils. Part 1. Influence of hydroxyapatite on metal exchangeability in soil, plant growth and plant metal accumulation

Abstract

In order to evaluate the possible use of hydroxyapatite (HA) as a soil additive for the in situ remediation of metal contaminated soils, the immobilizing capacity of this product was investigated. Three different concentrations of HA (0.5%, 1%, and 5% by weight (w/w)) were applied to a metal (Zn, Pb, Cu, Cd) and As contaminated soil originating from an old zinc smelter site in Belgium. After a three weeks equilibration period, exchangeable metal concentrations of the soils were determined using 0.1 M Ca(NO₃)₂ extraction. Test plants (Zea Mays cv. Volga and Phaseolus vulgaris cv. Limburgse vroege) were grown on all soils. Growth parameters were determined and mineral analysis (Cu, Zn, Pb, Cd, Ni, Mn, Mg, Ca, K, As and P) of plants was performed. Exchangeable metal contents in soil decreased with increasing HA application. Plant growth was partly restored on the 0.5% and 1% HA treated soils. However, at the 5% HA application rate growth was inhibited again. Plant mineral analysis showed that concentrations of `toxic' metals in the leaves of the test plants decreased after HA application. However, the uptake of essential trace elements also decreased and probably led to Mn-deficiency in maize. In bean, addition of 0.5% and 1% HA resulted in a gradual decrease of metal uptake. At the 5% application level an increase of Zn, Cu, and Ni uptake was observed compared to the 0.5% and 1% application rate. In contrast to metal uptake, As uptake was found to increase after HA treatment. The increased PO₄²⁻ concentration in the soil may be responsible for this. These results illustrate that HA application for the remediation of metal contaminated soils can be effective, but is not self evident. Strong immobilization of essential nutrients may lead to deficiency problems and mobilization of As may lead to an increased transfer to plants and animals and to an increased percolation of this element to the ground water.

Introduction

Mining, manufacture and disposal of metals and metal-containing materials inevitably cause soil contamination. One of the industries formerly causing severe contamination of soils was the non-ferrous metallurgical industry. Pyrometallurgical production processes lead to large emissions of metals such as Cd, Zn, Pb, Cu, etc. Due to the phytotoxicity of soils containing high levels of available metals, the natural vegetation cover might have disappeared to leave a bare site behind (Vangronsveld et al., 1991). Such bare sites pose a risk for the surroundings because the absence of vegetation facilitates lateral wind erosion of metal contaminated particles, and may enhance the volume of water percolating through the soil and eventually reaching the underlying ground water (Vangronsveld et al., 1991; Vangronsveld et al., 1995a, Vangronsveld et al., 1995b).

Remedial action may involve excavation and removal of the contaminated soil. Even though this seems like a logical solution, it is not feasible in many cases due to the vast size of the contaminated area and the high costs involved. A cheaper alternative for soil remediation is in situ stabilization of metals, rendering them immobile in order to reduce the risks of groundwater contamination, plant uptake, and exposure of other living organisms. Lower metal uptake by plants may result in the restoration of a vegetation cover that fixes and stabilizes the top layer of the soil, prevents wind erosion, and has a positive effect on metal percolation (Vangronsveld et al., 1991; Vangronsveld et al., 1995a, Vangronsveld et al., 1995b).

In situ stabilization of metals can be achieved by the application of additives to the soil. In the last few years, significant research has focused on this topic. Several soil additives have been tested: zeolites (Gworek, 1992a, Gworek, 1992b; Rebedea and Lepp, 1994; Chlopecka and Adriano, 1996a, Chlopecka and Adriano, 1996b), beringite (Vangronsveld et al., 1991, Vangronsveld et al., 1995a, Vangronsveld et al., 1995b, Vangronsveld et al., 1996; Mench et al., 1994a), and steel shots (Mench et al., 1994b; Sappin-Didier et al., 1997). Ma et al. (1993); Laperche et al. (1996)showed that the application of synthetic hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) led to immobilization of dissolved Pb in contaminated soils. In aqueous solutions, HA led to an immobilization of Al, Cd, Cu, Fe(II), Ni and Zn as well (Suzuki et al., 1981; Takeuchi and Arai, 1990; Middelburg and Comans, 1991; Ma et al., 1994b; Xu et al., 1994). HA seems therefore a promising soil additive for immobilizing metals in polluted soils.

Many studies have been conducted to understand the mechanism of this immobilization (Suzuki et al., 1981; Ma et al., 1993, Ma et al., 1994a, Ma et al., 1994b; Xu et al., 1994; Laperche et al., 1996). However, a chemical and physical evaluation of the efficiency of HA application as a remediation technique is not sufficient. Biological evaluation methods are necessary to assure that immobilization results in lower soil to plant transfer and phytotoxicity. Moreover, these methods can give indications about possible adverse effects (toxicity, deficiency of other elements) of HA application. Chlopecka and Adriano (1996aChlopecka and Adriano (1996b) showed that the addition of apatite (0.4% by weight) to a Zn, Cd and Pb polluted soil led to an increasing yield and lowered the Zn, Cd and Pb content in three week-old-maize plants (Zea Mays), in mature maize tissues (roots, young leaves, old leaves, stems, grains) and in barley (Hordeum vulgare). The addition of HA (0.6%, 1.16%, 1.74%, and 2.32% by weight) to Pb polluted soils led to a strong decrease of the Pb concentrations in shoots of sudax (Sorghum bicolor L. Moench) while the Pb concentrations in the roots decreased only at low application rates of HA. At the highest HA application rate, the Pb content in roots was strongly increased if compared to the untreated soil. No significant changes in the biomass of sudax shoots and roots were observed (Laperche et al., 1997). Except some data concerning the influence of HA on P and Ca uptake (Laperche et al., 1997), no data are available concerning the uptake of other essential nutrients. Besides, the influence of HA on the bioavailability of Cu has not been reported yet. At last, as there was no significant influence on shoot and root biomass of sudax at the application rates 0.6–2.32% (Laperche et al., 1997), it is not possible to conclude whether there is an eventual adverse effect at higher HA application rates.

In the present study HA was mixed at three levels (0.5%, 1% and 5% by soil weight) to a metal- (Zn, Pb, Cu, Cd) and As polluted soil originating from an old Zn smelter site in Belgium. After equilibration for three weeks, exchangeable metal contents were determined by means of extractions with 0.1 M Ca(NO₃)₂. Two different plant species (maize, Zea mays L. and bean, Phaseolus vulgaris L.) were cultivated on the soils and the effect of the different application rates on plant growth, metal accumulation and the uptake of other elements was determined. The main objectives of this study were: (1) to investigate the effect of increasing HA application rates on the amount of exchangeable metals in the soil, (2) to evaluate the effect of different HA application rates on plant growth and metal accumulation by plants, (3) to evaluate the effect of HA on the uptake of essential nutrients.