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

In order to evaluate the possible use of hydroxyapatite (HA) as a soil addi tive for the in situ remediation of metal contaminated soils, the immobiliz ing capacity of this product was investigated. Three different concentratio ns 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 sit e in Belgium. After a three weeks equilibration period, exchangeable metal concentrations of the soils were determined using 0.1 M Ca(NO3)(2) extracti on. Test plants (Zea Mays cv. Volga and Phaseolus vulgaris cv. Limburgse vr oege) were grown on all soils. Growth parameters were determined and minera l analysis (Cu, Zn, Pb, Cd, Ni, Mn, Mg, Ca, K, As and P) of plants was perf ormed. Exchangeable metal contents in soil decreased with increasing HA app lication. Plant growth was partly restored on the 0.5% and 1% HA treated so ils. However, at the 5% HA application rate growth was inhibited again. Pla nt mineral analysis showed that concentrations of 'toxic' metals in the lea ves of the test plants decreased after HA application. However, the uptake of essential trace elements also decreased and probably led to Mn-deficienc y in maize. In bean, addition of 0.5% and 1% HA resulted in a gradual decre ase 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 co ntrast 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 immobi lization of essential nutrients may lead to deficiency problems and mobiliz ation of As may lead to an increased transfer to plants and animals and to an increased percolation of this element to the ground water. (C) 1999 Else vier Science Ltd. All rights reserved.