Enhanced recovery of arsenite sorbed onto synthetic oxides by L-ascorbic acid addition to phosphate solution: calibrating a sequential leaching method for the speciation analysis of arsenic in natural samples

Stripping voltammetry capable of detecting 0.3 μg/L arsenate and arsenite was applied for speciation analysis of arsenic sorbed onto synthetic ferrihydrite, goethite at As/Fe ratio of 1.5 mg/g with or without birnessite after sequential extraction using 1 M phosphate (24 and 16 h) and 1.2 M HCl (1 h). Precautions to avoid oxygen were undertaken by extracting under anaerobic conditions and by adding 0.1 M L-ascorbic acid to 1 M NaH2PO4 (pH 5). Addition of L-ascorbic acid did not reduce As(V) to As(III). The recovery rate for As(III) using L-ascorbic acid for extraction (pH 5) but not for adsorption was 81% and 74% of total sorbed As, and was 99% and 97% of extracted As for ferrihydrite and goethite, respectively. Birnessite oxidized most As(III) during the adsorption procedure. L-ascorbic acid used both in adsorption and extraction procedures improved the recovery of As(III) to 79–94% for ferrihydrite–birnessite and 57–94% for goethite–birnessite systems with Fe/Mn ratios of 7, 70, 140 and 280 g/g.