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  • Organic acids inhibit the formation of pyromorphite and Zn-phosphate in phosphorous amended Pb- and Zn-contaminated soil.

Organic acids inhibit the formation of pyromorphite and Zn-phosphate in phosphorous amended Pb- and Zn-contaminated soil.

Journal of environmental management (2013-01-15)
F Debela, J M Arocena, R W Thring, T Whitcombe
ABSTRACT

Pyromorphite (PY) and some zinc phosphates (Zn-P) are very sparingly soluble minerals and hence can immobilize Pb and Zn in contaminated soils. However, mechanisms leading to the poor efficiency of PY and Zn-P formation in contaminated soils amended with P still remain unclear. We studied the influence of two low molecular weight organic acids (LMWOA) - oxalic acid and citric acid and diethylene triamine pentaacetic acid (DTPA) - in PY and Zn-P formation in a P-amended contaminated soil. Despite the high levels of metals (∼4% Pb and 21% Zn) in the study soil, the addition of up to 1% inorganic P transformed only up to 37% and 17% of the total Pb and Zn to PY and Zn-P, respectively. Semi-quantitative estimates from a linear combination fitting of X-ray absorption near edge spectra (LC-XANES fitting) showed that the formation of PY decreased from 37% to 3% of the total Pb in the presence of oxalic acid and the addition of 1% P. The reduced PY formation may be associated with the increase in organic-bound Pb from 9% to 54% and decrease in carbonate associated Pb from 42% to 12% with oxalic acid addition as indicated by a chemical sequential extraction (SE) technique. Citric acid seemed to have a less adverse effect in PY formation than oxalic acid. Our data also suggests both oxalic and citric acids have less adverse effects on the efficiency of Zn-P formation. From this study we conclude that the abundance of LMWOA in soil environments can be one factor contributing to the poor efficiency of P amendments practices to effectively immobilize Pb and Zn in metal contaminated soils.

MATERIALS
Product Number
Brand
Product Description

Supelco
Oxalic acid concentrate, 0.1 M (COOH)2 (0.2N), eluent concentrate for IC
Sigma-Aldrich
Oxalic acid, purified grade, 99.999% trace metals basis
Sigma-Aldrich
Oxalic acid, puriss. p.a., anhydrous, ≥99.0% (RT)
Sigma-Aldrich
Oxalic acid, 98%
Sigma-Aldrich
Oxalic acid, ReagentPlus®, ≥99%
Sigma-Aldrich
Sodium oxalate, ≥99.99% trace metals basis
Supelco
Sodium oxalate, reference material for titrimetry, certified by BAM, >99.5%
Sigma-Aldrich
Sodium oxalate, ACS reagent, ≥99.5%
Sigma-Aldrich
Sodium oxalate, puriss. p.a., ACS reagent, ≥99.5% (RT)
Sigma-Aldrich
Sodium oxalate, BioXtra