PRELIMINARY INVESTIGATION ON ARSENIC FRACTIONATION IN SOIL FROM OGOSTA RIVER FLOODPLAIN USING A SEVEN-STEP EXTRACTION PROCEDURE

Abstract

Arsenic (As) is a toxic element which can occur in increased concentrations mainly in areas affected by mining and ore processing activities. To assess the As fractionation in soils from the Ogosta River floodplain, a seven-step sequential extraction procedure (SEP) followed by As determination using ultrasonic nebulization inductively coupled plasma optical emission spectrometry (USN-ICP-OES) was applied. The SEP fractionate between the (1) ionically bound As; (2) strongly adsorbed As; (3) As co-precipitated with acid volatile sulphide, carbonates, Mn oxides, very amorphous Fe oxyhydroxides; (4) As co-precipitated with amorphous Fe oxyhydroxides; (5) extraction in 0,2M NH₄-oxalate buffer + ascorbic acid; (6) As associated with crystalline Fe oxides; (7) orpiment and remaining recalcitrant As minerals. No significant differences were found between the pooled amount of As concentrations in each extraction step and the total As concentration measured using a XRF spectrometer (recoveries rate of 90 –110%). Total As concentration in soils varied widely, in the range of 36 - 72300 mg kg^-1. The partitioning of As among the seven fractions in the six soil samples (%, medians and ranges) was: (1) 0.97 (0–4.8); (2) 12 (0–36); (3) 25 (12–44); (4) 8.7 (2.5–31); (5) 4.0 (0.2–25); (6) 34 (3.6–84); (7) 0.15 (0.02–1.1). Significant differences on As distribution in contaminated and uncontaminated soils were observed, the fractions of mobile species, were found to be predominant in highly contaminated soils in contrast to the low-As soils, where As contents were bound to the matrix.