Impacts of time delay in the pre-treatment of water samples on the physical and chemical contents of mine-polluted surface waters
Marja Liisa Räisänen, Geological Survey of Finland, P.O.Box 1237, FI-70211 Kuopio, Finland marja-liisa.raisanen(at)gtk.fi
Aqueous systems affected by mine-related pollution are commonly characterized by ferric-Fe (Fe3+) and Al compounds that precipitate rapidly and promote rapid decreases of metal concentrations in mixing zones where important pH and Eh (redox) gradients exist (Sánchez-España et al. 2011, Nordstrom 2011). Therefore, it is important to minimize changes in element solubility during sampling, transportation and storage, before chemical analyses.
The main target of the present study is to examine the impacts of delaying water sample pre-treatment (specifically, filtering and acidifying samples) on the results of sample chemical analysis. In addition, the study examines the changes in physical properties (pH, redox, oxygen concentration, electric conductivity) of samples in the laboratory one and two days after sample collection relative to in situ measurements. The water data examined in the present study was previously published in the Master thesis by Kankkunen (2008). The data consists of surface water sample analyses from the closed Kotalahti Ni-Cu mine area. The Kotalahti mine operated from 1959 to1987.
Study materials and methods
Water samples were collected at six points along a waterway. The upstream sample site receives overflow from a closed shaft and at the furthest downstream sample site stream water mixes with tailings impoundment seepage before discharging into a lake. Water samples were collected at the end of June, 2004 (Kankkunen 2008). A sample was collected in a 1L polyethylene bottle from each sampling point.
From the 1L sample a 100 ml subsample was filtered and acidified in the field immediately after sample collection. A second 100 ml subsample was collected from the initial 1L sample and pre-treated one day after sampling, and a third 100 ml aliquot was collected and pre-treated two days after the initial sample collection. Samples were stored and delayed pre-treatments were conducted in the GTK laboratory. Each 100 ml subsample was filtered through a single packed, 0.45 µm filter. Each filtered 100 ml-subsample was preserved with 0.5 ml Suprapur® nitric acid immediately after filtration. (Kankkunen 2008)
The 1L samples and subsamples were kept in a cool box in the field and during transportation and moved to a laboratory refrigerator during storage, prior to chemical analysis. Concentrations of 33 elements were determined using ICP-AES and MS-ICP methods. The present study presents data on 14 essential elements (Ca, Mg, Na, K, Al, Si, S, Fe, Mn, Ni, Cu, Co, Zn and Cd) relevant to mine-pollution.
The pH, redox, electrical conductivity (EC), oxygen concentration and oxygen saturation of water was measured with WTW field meters initially in situin the field, and in the laboratory concurrent with subsequent subsample collection. Field meter measurements taken in the laboratory were analysed from separate aliquots collected from the 1 L samples, concurrent with collecting each subsample for preservation at one and two day intervals after sampling. Sample pH was measured with a WTW pH340i/Set meter, redox with a Ag/AgCl electrode, EC with a Cond340i/Set meter and oxygen concentration and saturation with an Oxi330/Set meter. (Kankkunen 2008)
Results and discussion
The greatest change in redox potentials and oxygen concentrations of the sampled waters was observed two days after sample collection, and somewhat smaller changes were observed one day after sampling. The increase in redox potentials after one day ranged from 15 % to 86 % and after two days from 16 % to 88 %. The greatest increase in redox potential (86-88 %) was observed at the point (Di-2) which receives tailings seepage and which was slightly reductive when measured in situ. This sample became increasingly oxidative in the laboratory, one and then two days after sampling. The greatest change in oxygen concentrations occurred in the two most upstream sample sites, closest to the underground mine water discharge point (shaft overflow). In contrast, pH and EC changed less with time after sample collection.
Soluble concentrations of S and base cations (Ca, Mg, Na, K) remained similar, regardless of the pre-treatment timing relative to sampling (Table 2). The concentrations ranged within the precision and accuracy of the analytical methods (< 2-5 %). The combined solubility of base cations and S was the major control on EC and to some extent on pH. When the solubility of these ions decreases, solution pH increases slightly, likely through neutralization reactions concurrent with the precipitation of alkaline or alkaline earth hydroxides. In this study case, these relationships appear to be independent of pre-treatment timing.
The majority of samples revealed a decrease in Fe concentration when pre-treatment was done one or two days after sampling. With the exception of one sampling point, relative to samples pre-treated in situ, Fe concentrations decreased from 50 to 90% in sample pre-treated one to two days after sampling. The soluble Fe concentration in the exception mentioned above was initially very low (0.1 mg/l) for the in situ pre treatment subsample, doubled in the subsample with underwent pre-treatment after one day, and remained approximately constant in the sample that underwent pretreatment after two days. These results show that the precipitation of Fe can continue after sample collection, or alternately, Fe can be released from Fe bearing solids (precipitates), if samples are not pre-treated immediately. The precipitation of Fe that continues prior to sample pre-treatment is mainly caused by Fe oxidation during transport and storage, as supported by the observed increases in redox potentials and oxygen concentrations with time after sampling (Fig. 1, See also Tables 1 and 2). However, the oxidation of Fe resulted in no detectable decrease in pH (cf. Nordstrom 2011). This is assumed to be caused by concurrent neutralizing reactions (acid consumption by organic matter and/or precipitation of alkaline or alkaline earth hydroxides such as gypsum) which likely buffered against the drop in pH otherwise expected to result from Fe oxidation (e.g. Bachmann et al. 2001). Analysis of solid phase chemistry from oxidized water samples would allow for a more complete understanding of reaction pathways prior to pre-treatment.
In addition to Fe, the timing of the pre-treatment impacted the solubility of Al (Table 2). The solubility of Al was observed to either increase or decrease, depending on sample point, relative to subsamples pre-treated immediately upon sample collection (Table 2). The change in Al solubility varied from 20 % to 70 % and could not be linked to any measured parameter, perhaps due to near-neutral pH (See Sànchez-Espanã et al. 2011). Among chalcophile metals the pre-treatment timing affected the solubility of Cu and Zn, while there was no discernible effect on Ni and Co. Concentrations of Zn and Cu varied from <5 % to 40 %, depending on the sample point. In contrast, the change in Ni and Co solubility was minimal (< 6 %), and was within the precision and accuracy of the MS-ICP analysis.
Table 1. Physical properties of mine-polluted waters measured in situ immediately after sample collection, and in the laboratory one and two days after sampling. Keys for sample codes: Di-1 (S) surface water receiving overflow from the closed shaft, Pond1 and Pond2 downstream surface waters from excavation sites, Di-2 (T) seepage water discharged via a pipe from the tailings impoundment, drain surface water downstream, before discharging into a lake.
Table 2. Chemical properties of mine-polluted waters measured in situ immediately after sampling, and in the laboratory one and two days after sample collection. See keys for samples in Table 1.
Figure 1. Solubility of Fe versus (a) oxygen concentration (mg/l) and (b) redox potential (mV) in samples pre-treated in situ (in the field), one day and two daysafter sampling, Kotalahti Ni-Cu mine, eastern Finland. Key: Di-2 (T) refers to seepage water from the tailings impoundment (See also Table 2).
The timing of water sample pre-treatment (i.e. filtering and preservation with acid) in near-neutral mine-polluted waters had the greatest impact on the solubility of Fe in solution, and lesser, but discernible impacts on the solubility of Al, Zn and Cu. The changes in Fe solubility were primarily attributed to observed increases in oxygen concentration and redox potential during transportation and storage of the water samples before analyses, despite efforts made to transport and store samples at cool temperatures.
On the basis of these results, it is recommended to pre-treat water sample intended for ICP determinations in situ, immediately after sampling.
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