Evaluation of icp-oes method for arsenic determination in sterile dump material

Annotation

A development and validation of an analytical method for the determination of arsenic in waste material from barite recovery by floatation is presented. The method is based on aqua regia open digestion sample pretreatment and dual view inductively coupled plasma atomic emission spectrometry. A soil certified reference material was used to study the efficiency of wet digestion procedure applied to sterile dump material. The sensitivity, accuracy, and precision were determined. The detection limit was 0.80 mg/kg dry material. The developed method was applied for the determination of arsenic in sterile dump material from closed barite mine in the region of Tamita-Suceava, Romania.

Introduction

Ore-processing activities generate an enormous quantity of solid waste in a local as well as in a worldwide scale. Intensive research has been made in an attempt to estimate the short-time and a long-time risk to the environment [1-4]. Management of current and historical waste from the mining industry is aimed at preventing or reducing the adverse environmental impacts and described in a line of international regulations [5, 6].

The migration of heavy metals, metalloids and other harmful substances from waste deteriorates the quality of soil, surface and groundwater. It affects biodiversity and human health directly and regular monitoring of the waste and surrounding environment state is required. The following hazardous metals and their compounds should be strictly monitored: arsenic, lead, cadmium, chromium, copper, mercury, nickel, zinc [7-9]. To ensure quality of chemical analysis and low limits of quantification of harmful and hazardous compounds a number of requirements have been imposed to analytical methods [9-13].

The region of Tarnita-Suceava, Romania is known as an intensive mining area. The long-term activity of a barite mine in this region has resulted in an enormous amount of sterile dump material, collected in several dumps and tailing ponds. The deposited waste exerts negative impact on the environment [1, 14] and its ecological fate should be regularly monitored by determination of the chemical composition of sterile dump material and surrounding soils. As previous studies reviled the sterile dump material and surrounding soils contain high levels of arsenic and heavy metals [4].

ICP-OES determination is well known for the multicomponent determination of soil components and some standard methods are described. An aqua regia wet digestion is the most used sample preparation method for metal determination by ICP-OES multi-element analysis of geological samples, soils and mine waste [10-12]. However, the appropriate analytical procedure for sample preparation and ICP-OES measurements should be optimized and validated in each laboratory.

This report is aimed at estimation of analytical characteristics of ICP-OES determination of total arsenic in sterile dump material after wet sample digestion. Efficiency of wet sample digestion with aqua regia was evaluated. The extraction effectiveness of arsenic is verified by analyzing certified reference soil material. Limits of detection and quantification, sensitivity, accuracy, repeatability are estimated to confirm the applicability of the optimized method for arsenic determination in sterile dump material. The method was applied for determination arsenic in a waste sterile dump material and surrounding soils nearby closed barite mine in Tarnita-Suceava area, Romania.

Experimental

Reagents and equipment. An arsenic standard solution (As in 5% HNO3) 1OOO ± 3 mg/L “VHGLabs” were used. HNO3 65% and HCl 37% from Thermo Fisher Scientific analytical grade were used for samples digestion. A ERA Metals in soil certified reference material was used for the quality control of metals determination. A Prodigy high dispersion ICP-OES dual view spectrometer (Telledyne Leeman Labs USA), equipped with a dual view torch, cyclonic spray chamber, was used.

Extraction procedure. Solid waste samples from sterile dump were dried at 110 ⁰C for 2Һ and homogenized after cooling. Approximately 0.4 g (± 0.0002 g) of sample were weighted and transferred to digestion vessel. A 15 ml aqua regia was added. The beakers were covered with watch glass and heated on a hot plate at 95°C without boiling. The obtained solutions were filtered, collected in 50 mL flasks and diluted with distillated water.

ICP-OES determination. Due to the nature of current sample matrices and digestion procedure, the operating conditions of ICP-OES spectrometer recommend for metal in soils determination were used. The operating conditions were: coolant gas 18 L/min, auxiliary gas 0.5 L/min, nebulizer gas 34 psi, RF power 1.2 kW, pump rate 1.2 mL/min, sample uptake time 30 sec, integration time 30 sec axial view. High purity Ar 99.999% supplied by SIAD BG was used to sustain plasma and as a carrier gas. Two- point background correction and three replicates were used to measure the analytical signal. An emission line at 193.696 nm, free from spectral interference, was chosen for acquiring the emission intensity of As.

Sequential extraction of arsenic. Extraction step 1: 2.5 g soil is mixed with 10 mL IM Mg(NOs)?, the mixture was sonicated for 30 min, the solution was centrifuged for 15 min at 6000 r/min and the supernatant was separated, in the washing step 5 mL d. H2O was added to the solid, sonicated for 10 min, centrifuged for 15 min. Both supernatant were collected and diluted to exact volume. Extraction step 2: 10 mL of 0.1 M NH2OH.HCI were added to the residue from step 1, sonicated for 15 min at 50 ⁰C. The extraction mixture was centrifuged for 10 min at 6000 r/min the supernatant was separated and the residue was washed with 8 mL d. H2O, after 3 min sonication and 10 min centrifugation. Both supernatants were collected and diluted to exact volume with d. H2O. Extraction step 3: 10 mL of 0.25 M NH2OH.HCI+O.25 M HCl were added to the residue from step 1, sonicated for 20 min at 50 ⁰C. The extraction mixture was centrifuged for 10 min at 6000 r/min. the supernatant was separated and the residue was washed with 8 mL d. H2O, after 3 min sonication and 10 min centrifugation. Both supernatants were collected and diluted to exact volume with d. H2O. Extraction step 4: 10 mL of 0.2 M (ҺШд^СгСЕ+О.І M H2C2O4+O.I M citric acid the residue from step 3, sonicated for 20 min at 100 ⁰C (water bath). The extraction mixture was centrifuged for 10 min at 6000 r/min. The supernatant was collected and diluted to exact volume with d. H2O. Extraction step 5: the residue was dried at 120 ⁰C and 0.25 g residues (dry bases) were taken for the last extraction step. 12 mL aqua regia HNO3+HCI is added to the sample in two portions. The sample was boiled for 25 min in a beaker covered with watch glass. The extract was filtered and diluted with distilled water. The obtained extracts were analyses by ICP-OES.

Results and discussion

Optimization of the extraction procedure. Extraction of arsenic from sterile dump material was based on a well-known aqua regia open digestion method. In this study the number of extraction steps, extraction time and volume of aqua regia were optimized. Two set of experiments were made: (1) extraction of arsenic from sterile dump material at different extraction times and (2) evaluation of extraction efficiency at optimized time by analyzing reference soil material. In the first set of experiments sterile dump material (in duplicate) was digested for 20, 40, 60 and 80 min.

The results are presented in Table 1 The total heavy metal content is also presented. As can be seen from the Table a digestion time of 20 minutes was enough to fully extract hazardous metals and arsenic from sterile dump material. Increasing the time of extraction didn’t affect the total content of metals. The increase in arsenic concentration was only 2% for additional 60 min digestion. One stage 20 min extraction was further applied in this study. However, it should be noted that samples more than 0.4 g would require longer digestion procedure.

In the second set of experiments, the efficiency of aqua regia open digestion method was verified by analyzing soil reference material. The mine waste composition was close to the composition and structure of the soil and a certified referenced soil material was used for accuracy estimation. After extraction and ICP-OES measurement the results were compared with certified value and recovery was calculated. The results are presented in Table 2. Recovery acceptance criteria depend on the nature of samples; method of digestion and concentrations ranges [15]. The obtained results were within the quality control acceptance limits of the certified material.

Table I Concentration of arsenic and total heavy metals in sterile dump material (mg/kg) at different extraction time (N=2, P=95%)

Table 2 Determination of arsenic in reference soil material by ICP-OES after aqua regia digestion (number of samples 5)

Analytical characteristics of the proposed method

Analytical characteristics of ICP-OES method were studied using multipoint external standard calibration method. Each standard solution was analyzed in triplicate and a mean intensity of emission was calculated. The response curve (I = 0.035 + 7.10x10 ⁵. C) was linear in the studied concentration range 1-10 mg/L with correlation coefficient 0.9999. Limits of detection (LOD) were estimated by analyzing blank solution in triplicate. Blank was subjected to the extraction and measuring procedures. LOD was calculated as LOD = xb+3.Sb, where хь and Sb were intensity of signal of the blank sample and its standard deviation [13, 19]. Limit of quantification (LOQ) was calculated according to the equation LOQ = 3.L0D [13, 19]. LOD and LOQ were calculated in mg/kg of the original solid samples for 1 g digested sample and the final volume of 100 mL of obtained solution after metal extraction. The following LOD and LOQ were obtained: 0.8 and 2.4 mg/kg, respectively. The reproducibility of the blank was RSD= 14 %.

The accuracy of the proposed method was estimated by a determination of recovery of spiked sterile dump material. Three dry sterile dump samples, taken from different dumps, were spiked with standard arsenic solution. The samples were treated according to the described extraction procedure followed by ICP-OES determination before and after spiking. Each sample was analyzed in duplicate. The recovery was calculated as [13]:

Я = _xl00

f

Where Ca+spike is the concentration of analyte after spiking, C_a - concentration of analyte before spiking and C_spike - concentration of spiked analyte. All concentrations are recalculated in mg/kg dry weight. The results are presented in Table 3. The expanded uncertainty was calculated as a combined uncertainty (SD) multiplied by a coverage factor к = 2 [16]. At normal distribution this value of the coverage factor corresponds to a confidence level of approximately 95%. The uncertainties include both stages: sample preparation and measurement. The largest contribution to the combined uncertainty had the sample preparation step. In order to fully estimate the uncertainty of the method 9 samples of the same type were analyzed within 6 moths. The results are presented in the Table 3.

Evaluation of icp-oes method for arsenic determination in sterile dump material

Table 3 Results from determination of arsenic in sterile Dump material by ICP-OES method after aqua regia digestion

Arsenic species distribution in soil fractions from Tarnita-Suceava, Romania region.

For studying the currents state of Tarnita environment two soil samples and one sample from sterile dump material were taken. There are several sterile dumps created during the barite mine operation. Tarnicioara River passes between two of the sterile dumps. Samples were collected from the surface horizons (where possible, from the first 20 cm). The sampling sites are presented on Fig. 1. The samples are denoted as follows: soil sample Tl was taken at 30 m distance from the sterile dump and near the river in a close proximity of the sterile dump (Figure 1 A); sample T2 is a material from sterile dump (Figure IB); sample T3 is soil from the river bank (Figure 1 A).

A modified procedure for sequential extraction of arsenic species was applied in attempt to estimate the arsenic species distribution in soil fractions. The procedure is based on the sequential extraction procedure proposed by D. Arenas-Lago et al. [18]. hi this study the extraction procedure was modified by applying sonication during extraction and washing steps [19]. By applying sonication the leaching of toxic metals and metalloids under stress conditions could be evaluated. The results could be used for determining the mobility of arsenic species after short stress impact exerted by natural or human activity related factors on the sterile dump. The distribution in the soil is strongly correlated with arsenic species availability and possible toxicity. The extracts obtained were analyzed applying the validated ICP-OES method. The results are

presented on Figure 2. As can be seen from the Figure the main part of arsenic is associated with iron oxides, higher quantity to the crystalline iron oxides. The main fraction was extracted during the stage 4: 94 % (Tl); 56 % (T2) and 98% (T3), no arsenic was extracted during the stages 1 and 2. The determined content in all studied samples is far above the threshold limits stated by ICRCI, 1987: 40 mg/kg dry soil.

Conclusions: A protocol for determination of arsenic in sterile dump material from barite mine was proposed. The method was based on an acid sample digestion and ICP-OES analysis of the obtained extracts. The method characteristics were evaluated and results showed that the proposed protocol is suitable for analysis of specific samples from sterile dump material with recovery close to IOO % and precision of 5%. Extended uncertainty was also estimated. The proposed method was applied for determination of total arsenic and its distribution in soil fractions and sterile dump material from Tarnita region in Romania.

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