Development, optimisation and application of diffusive gradients based passive sampler monitoring of arsenic, selenium and mercury in waters
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Date
2019
Authors
Maputsoe, Xolisiwe
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Abstract
The diminishing levels of untainted fresh water sources suggest the need for more stringent monitoring approaches. Traditionally, grab sampling is the commonly used technique; however, the large number of associated limitations has led to the development and advancement of alternative and complimentary techniques such as passive sampling. One of the most widely used passive samplers is diffusive gradients in thin-films (DGT). DGT was designed for the in situ measurement of labile trace metal species in natural water. It operates on the fundamentals of an integrative passive sampler whereby analytes accumulate linearly with time, against a concentration gradient. Since the accumulation of analytes occurs at a definable rate DGT is able to provide time integrated concentrations of pollutants. Additionally, because analytes enter the passive sampler exclusively via diffusion, it provides information on the bioavailable fraction. DGT is made up of two functional layers: a diffusive layer and a binding layer. The binding layer can be modified to suit the target analytes.
In recent years polyethylenimine (PEI) has attracted a lot of attention as an adsorbent for metals. Cross-linked polyethylenimine (CPEI), the insoluble version of PEI showed great potential as a remediation and analytical sorbent. In particular, phosphonated and sulphonated CPEI showed high efficiency in the removal of arsenic, selenium and mercury. Therefore, sulphonated and phosphonated CPEI polymers were considered as a DGT binding layer resins. In addition, DGT sample holders used in this study were manufactured from a 3.0 cm diameter polytetrafluoroethylene (PTFE) rod. The sample holder design and dimensions were similar to commercial DGT. Modifications were only made on the DGT cap, which would allow the same performance as commercial DGT but with the added advantage of being re-usable.
CPEI was modified using two methods; in the first method, CPEI underwent a sulphonation reaction using 3-chloropropanesulfonyl chloride to form sulfonated cross-linked polyethyenimine (SCPEI). In the second, phosphonation proceeded by reacting CPEI with phosphoric acid and formaldehyde to form phosphonated cross-linked polyethyenimine (PCPEI).
Due to the polymeric nature of the sulphonated and phosphonated CPEI resins, they had to be optimised as DGT binding layers, and two considerations had to be made: firstly, the optimum ratio of SCPEI and PCPEI in the resin mixture had to be determined. Secondly, how the polymeric resin mixture would be assembled into a binding layer. Binding layer assembly was carried out using two methods. In the first method the resin mixture was embedded in an agarose gel solution. In the second, 0.4 and 0.8 g of SCPEI-PCPEI resin mixtures in their loose polymer form were used. This resin mixture consisted of 80% SCPEI and 20% PCPEI of total resin mass used. The DGT sampler was also calibrated under laboratory conditions to determine the influence of the following parameters: pH, sample concentration and turbulence. The passive sampler was further calibrated using environmental samples in laboratory based experiments: the passive samplers were deployed in spiked and un-spiked dam water, dissolved efflorescent crust as well as spiked acid mine drainage water. The optimised DGT passive samplers were then deployed in Fleurhof Dam for 12 days.
Results from binding layer optimisation show that mixing 80% of sulphonated CPEI and 20% phosphonated CPEI per total mass showed better adsorption of arsenic, selenium and mercury. In addition, unlike commercial DGT, the passive sampler with the resin mixture incorporated in agarose showed grossly reduced capacity, especially for deployments longer than 9 days. The maximum capacity was achieved using 0.8 g of SCPEI-PCPEI in the loose polymer form. Calibration results of the DGT sampler show that it can operate in the pH range of natural water under stagnant conditions. The passive sampler also showed better performance in dilute solutions, preferably less than 0.5 mg L-1.
Results from the spiked and un-spiked dam water samples show that arsenic and selenium were accumulated linearly with time by DGT. Mercury accumulation was linear only in the spiked sample; it did not follow a specific trend in the un-spiked dam water solution. In dissolved efflorescent crust, only arsenic showed linear mass accumulation with time. Conversely, no selenium and mercury were detected in the passive samplers. The same trend was observed for samplers deployed in spiked AMD water. All deployments in environmental samples
revealed that the SCPEI-PCPEI resin mixture can also accumulate other metal ions. In both the dissolved crust and AMD water deployments iron concentrations were elevated. This resulted in the formation of colloid particles within the passive sampler binding layer. These particles behaved as secondary adsorption sites, leading to the non-specific accumulation of analytes. During field deployments in dam water, the DGT passive sampler was able to accumulate arsenic linearly but showed reduced capacity for selenium and mercury since there was no increase in mass over time. Field deployments were able to highlight that SCPEI-PCPEI resin mixture is highly subjected to reduced selectivity and capacity for the target analytes in the presence of competing ions. This could be due to the high uptake of metal ions found in the dam water. Despite these challenges, SCPEI-PCPEI based DGT could be used for successful in situ measurement of labile arsenic in environmental waters. Measurement of selenium and mercury were however, unsuccessful.
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A Thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the Degree of Doctor of Philosophy
March 2019