Factors influencing the efficiency of arsenic extraction by phosphate



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Texas A&M University


Extraction with sodium phosphate has been used as a method of accessing arsenic in soils. Arsenic extraction efficiency by phosphate from rice-paddy soils of Bangladesh usually has been low and highly variable between soils. The major objectives of this study were to examine the relationships between phosphate-extractable arsenic and soil iron-oxide composition and to investigate the experimental factors which might influence arsenic-extraction efficiency from rice-paddy soils of Bangladesh by phosphate. Statistical analysis of approximately 500 surface soils from Bangladesh indicated that phosphate-extractable arsenic was well correlated with total soil arsenic (r2 = 0.832) and oxalate-extractable arsenic (r2 = 0.825), though extraction efficiency varied widely (5

  • 54 % of the total soil arsenic). The thanas with the lowest arsenic contents generally also had the soils with the lowest arsenic-extraction efficiencies. Quantity of phosphate-extractable arsenic was weakly correlated with the soil iron-oxide content, but extraction efficiency (i.e., the proportion of phosphate-extractable arsenic to total soil arsenic) was not correlated with any iron-oxide parameter. Arsenic extraction was strongly influenced by reaction variables such as sample grinding, phosphate concentration, principal counterion, reaction pH, and reaction time. The extraction efficiency was impacted by the influence of these individual factors on reaction kinetics and accessibility of arsenic adsorption sites for ligand exchange by phosphate. A portion of the arsenic was readily exchanged during the first few hours of extraction, followed by a much slower subsequent extraction. These results indicate that some of the arsenic is easily exchanged, but for a substantial portion of the arsenic, either the reaction kinetics is very slow or the sites are not accessible for reaction with phosphate. Extraction by phosphate is a useful procedure for the assessment of readily ligand-exchanged arsenic.