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

• Published in: Journal of environmental management 2013-02, Vol.116, p.156-162
• Main Authors: Debela, F., Arocena, J.M., Thring, R.W., Whitcombe, T.
• Format: Article
• Language: English
• Subjects: absorption; Acids; Acids - chemistry; adverse effects; Animal, plant and microbial ecology; Applied ecology; Biological and medical sciences; Chemicals; citric acid; Citric Acid - chemistry; Conservation, protection and management of environment and wildlife; Ecotoxicology, biological effects of pollution; Environmental management; Fundamental and applied biological sciences. Psychology; General aspects; Heavy metals; lead; Minerals; Minerals - chemistry; Molecular weight; Organic acids; Organic Chemicals - chemistry; oxalic acid; Oxalic Acid - chemistry; phosphates; Phosphates - chemistry; Phosphorus; Phosphorus - chemistry; polluted soils; Pyromorphite; Rhizosphere; Soil contamination; Soil degradation; Soil Pollutants - chemistry; Terrestrial environment, soil, air; X-radiation; zinc; Zinc Compounds - chemistry; Zinc-phosphates
• ISSN: 0301-4797; 1095-8630
• DOI: 10.1016/j.jenvman.2012.11.037

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. ► LMWO acids are found to inhibit the formation of PY and Zn-P in soils. ► Oxalic acid reduces PY and Zn-P formation more than citric acid. ► Routine soil analytical techniques are insufficient to conduct metal speciation studies. ► The use of advanced XAS techniques is recommended.