3.4 Speciation Investigations
3.4.3 Selective Organic Phase Extraction of As III from Soil
The arsenic speciation method utilized in this investigation was chosen because it was desirable to quantitatively speciate all of the arsenic present in selected soil samples using a simple procedure. The method involves extracting the arsenic from the soil with concentrated hydrochloric acid and to speciate using solvent extractions. The method works on the premise that AsIII can be selectively extracted into an organic phase from a strongly acidic phase and is then back-extracted into water for analysis [29,264-265]. It has been shown that inorganic arsenic in solution is most stable in its hydrolysed form [263], however, in the presence of an excess of hydrochloric acid, chlorination of arsenic will occur, yielding arsenic trichloride and arsenic pentachloride. It has also been shown that arsenic trichloride is a covalent molecule while arsenic pentachloride most likely exists as the complex ions, [AsCl4]+ or [AsCl6]¯ [263]. The arsenic
trichloride can be extracted into an organic phase such as chloroform or benzene, while AsV is excluded owing to its ionic properties. The AsIII contained in the organic phase can be easily recovered by back extraction with water, since inorganic arsenic is most stable in solution in its hydrolysed form. When the AsIII comes into contact with water, hydrolysis occurs, excluding the arsenic from the organic phase [32].
This arsenic speciation method was first implemented to standardize an arsenite sample and then used to determine the amount of AsIII sorbed onto soil at different pH levels.
3.4.3.1 Method for the Standardisation of Arsenite Sample
Three separate aqueous stock solutions were made-up containing the following arsenic species: (i) 76.7 mg L-1 AsIII as NaAsO2, (ii) 55.1 mg L-1 AsV as Na2AsO4.7H2O, and (iii) 50% v/v mixture of the AsIII and AsV stock solutions. Analytical grade milli-Q water was used to dilute each of the solutions. Determination of AsIII was conducted in the following way. A 10 mL aliquot of each of the stock solutions was transferred into three separate 100 mL separating funnels and 80 mL of 10 M HCl (high purity,
Mallinckrodt Chemical Company, U.S.A.) was then added to each separating funnel to adjust the acid concentration to greater than 9 M. This was followed by extraction of the AsIII into chloroform with 4 × 10 mL washings. At that stage the strongly acidic aqueous phase (i.e. top layer) was discarded. The AsIII was then back-extracted from the organic phase into 2 × 20 mL aliquots of water and diluted to 100 mL. Each of the three samples were analyzed using an ICP-AES, Perkin Elmer Optima 4300 DV with CETAC Technologies ASX – 500 model No. 510 auto-sampler). The ICP-AES analysis used an Rf power of 1400 W, outer gas flow of 15 L min-1, intermediate gas flow of 0.5 L min-1, nebulizer gas flow of 0.65 L min-1 using argon gas, sample flow rate of 1.5 mL min-1, instrument operating pressure at normal atmospheric pressure (760 mm Hg) and the temperature was maintained at 21°C.
The AsTot was also determined by running the three aqueous stock solutions individually through the ICP-AES. The concentration of AsV was calculated by subtracting the AsIII concentration from the AsTot. A percent difference from the AsTot was calculated by taking the difference of AsTot from AsIII and then dividing by the AsTot concentration. The percent difference is the amount of arsenic which most likely relates to the oxidative impurities present in the sample container.
3.4.3.2 Method for Arsenite Sorbed onto Soil at Different pH Values
The first part of this experiment involves allowing the sorption of arsenic onto soil samples. This was achieved using the same three stock solutions that were prepared in
Section 3.4.3.1. The pH of the stock solution was adjusted by pouring 50 mL of the NaAsO2 aqueous stock solution into 5 separate beakers and adjusting the pH in each of the beakers to pH values of approximately 2, 4, 6.5, 9 and 12 respectively using 0.1 to 1.0 M H2SO4 and 0.1 to 1.0 M NaOH. Where appropriate, throughout this work, pH measurements were made repeatedly to ensure that the pH of the medium remained constant.
A 5 g soil sample (light clay - S2) was placed into six separate plastic 50 mL centrifuge tubes. Five of these centrifuge tubes containing the pre-weighed soil sample were made up to the 50 mL mark using the pH-adjusted stock solutions. The sixth centrifuge tube was made up to the 50 mL mark with analytical grade milli-Q water (blank H2O). The contents of each of the six tubes was mixed on a rotating mixer for 30 min and then left to soak for 7 days. The samples were centrifuged for 15 min at 3000 r.p.m. and
decanted. The supernatant liquid from the decanting was retained for later use. Each soil sample was washed with 20 mL milli-Q water, mixed for 5 min on a rotating mixer, centrifuged for 15 min at 3000 r.p.m. and decanted. The washing procedure was
repeated a second time.
The next part of the experiment involved the extraction of AsTot from the soil. This involved treating each of the six samples with a 20 mL aliquot of 10 M HCl, and shaking it vigorously for 30 min. Each sample tube was centrifuged for 5 min at 3000 r.p.m. and poured into a 100 mL volumetric flask. The AsTot extraction procedure was repeated two more times (i.e. another 2 × 20 mL of 10 M HCl). The soil sample was then washed into filter paper (Whatman 44) with milli-Q water and diluted to the 100 mL mark. The concentration of the AsIII species was determined in each sample by taking a 10 mL aliquot of the AsTot extract and transferring it to a 100 mL separating funnel where 80 mL of 10 M HCl was added. The AsIII was extracted with 4 × 10 mL washings of chloroform. The acidic aqueous phase was discarded. The AsIII was then back-extracted from the organic phase using 2 × 20 mL aliquots of milli-Q water and diluted to the 100 mL mark in a volumetric flask. Each of the 6 samples was then analysed by ICP-AES.
The final part of the experiment involved determining the AsIII concentration in the supernatant liquid for each of the six samples. The AsIII concentration was determined by taking a 10 mL aliquot of each of the supernatant liquids and transferring each
aliquot to a 100 mL separating funnel. To each of the tubes, 80 mL of 10 M HCl was added. The AsIII was extracted using 4 × 10 mL washings of chloroform. The acidic aqueous phase was discarded. The AsIII was back-extracted from the organic phase using 2 × 20 mL aliquots of milli-Q water and diluted to the 100 mL mark in a volumetric flask. Each of the supernatant liquid samples was then analysed by ICP- AES.
The whole procedure was repeated two more times, changing the original 76.7 mg L-1 AsIII as NaAsO2 aqueous stock solution to: (i) the previously prepared aqueous stock solution of 55.1 mg L-1 AsV as Na
2AsO4.7H2O; and (ii) the previously prepared aqueous stock solution containing 50% v/v mixture of the AsIII and AsV stock solutions.
In Sections 3.2, 3.3 and 3.4, where time and resources permitted, replicates and reference soil material were used for QA/QC purposes. Results for QA/QC have been included on the appended CD. The preparation of fresh stock solutions of particular arsenic species was carried out for each experimental section in order to minimize possible transformation of arsenic between different forms.