Ecotoxicity of silver nanomaterials in the aquatic environment: a review of literature and gaps in nano-toxicological research.

There has been extensive growth in nanoscale technology in the last few decades to such a degree that nanomaterials (NMs) have become a constituent in a wide range of commercial and domestic products. With NMs already in use in several consumer products, concerns have emerged regarding their potential adverse environmental impacts. Although research has been undertaken in order to minimise the gaps in our understanding of NMs in the environment, little is known about their bioavailability and toxicity in the aquatic environment. Nano-toxicology is defined as the study of the toxicity of nanomaterials. Nano-toxicology studies remain poorly and unevenly distributed. To date most of the research undertaken has been restricted to a narrow range of test species such as daphnids. Crabs are bio-indicators that can be used for toxicological research on NMs since they occupy a significant position in the aquatic food chain. In addition, they are often used in conventional ecotoxicological studies due to their high sensitivity to environmental stressors and are abundantly available. Because they are benthic organisms they are prone to contaminant uptake and bioaccumulation. To our knowledge the crab has never been used in nano-toxicological studies. In this context, an extensive review on published scientific literature on the ecotoxicity of silver NPs (AgNPs) on aquatic organisms was conducted. Some of the most common biomarkers used in ecotoxicological studies are described. Emphasis is placed on the use of biomarker responses in crabs as monitoring tools, as well as on its limitations. Additionally, the gaps in nano-toxicological research and recommendations for future research initiatives are addressed.

A review of mercury pollution in South Africa: current status.

The present paper is a review on the status of mercury (Hg) as a pollutant in South African aquatic ecosystems. Spatial patterns of Hg distribution and bioaccumulation in water resources were investigated by collecting and analyzing multimedia samples for physiochemical and Hg-species determination from 62 sampling sites. The data presented showed a wide range in concentrations, which was expected given the array of environmental parameters, water chemistry and sources of Hg. Generally, higher Hg concentrations were measured in environmental compartments impacted by the major anthropogenic Hg sources which, in South Africa, are largely represented by emissions from coal-fired power stations (i.e. Olifants and Upper Vaal WMAs) and artisanal gold mining (i.e., Inkomati WMA). Ancillary water quality parameters (e.g. pH, temperature, DOC, EC and nutrients) were measured and regressed with the measured Hg concentrations to determine which environmental parameters most influenced regional Hg concentrations. The TotHg (sed) and DOC concentrations were identified as important factors controlling TotHg (aq), while TotHg(sed) were correlated to TotHg (aq). This result is indicative of the combined effects of sediment settling and resuspension in the aquatic environment. In contrast, MeHg (aq) was not correlated to DOC. MeHginvert were correlated to MeHg (sed), while MeHg (fish) were correlated to MeHg (aq) and water quality variables (chlorides--Cl(-) and electrical conductivity--EC). A steady progress has been made in Hg research in South Africa. However, despite the substantial knowledge about Hg toxicity, there are still considerable knowledge gaps on the fate and transport of Hg. Hence, further environmental and human health studies are proposed.

Mercury concentrations at a historically mercury-contaminated site in KwaZulu-Natal (South Africa).

INTRODUCTION: A mercury (Hg) processing plant previously operating in KwaZulu-Natal Province (South Africa) discharged Hg waste into a nearby river system causing widespread contamination since the 1980s. Although the processing plant ceased operation in the 1990s, Hg contamination (due to residual Hg) remains significant. Previous studies in the area since the plant's closure have found elevated Hg concentrations in fish, and that these concentrations were as a direct consequence of widespread contamination of the Hg processing plant operations conducted between the 1980s and 1990s. OBJECTIVES: This study aimed at investigating the impacts of residual Hg almost 20 years after the plant's closure. METHODS: Water, sediment and biota (invertebrates and fish) were collected in water resources in the vicinity of the processing plant to determine the Hg concentrations in these compartments, as a proxy for assessing the extent to which residual Hg that is reintroduced to the water column becomes bioavailable to biota. For water and sediment samples, higher total mercury (TotHg) and methylmercury (MeHg) concentrations were measured at sampling sites immediately downstream of the Hg processing plant when compared to the upstream sites, while concentrations decreased with distance from the plant. Fish MeHg concentrations measured just below the US EPA guideline for Hg in fish muscle tissue. RESULTS: The results show that the historically Hg-contaminated river system is a potential Hg pollution source due to the residual Hg present in sediment. Any dredging of sediment as a form of remediation in the Mngceweni River is not recommended; however, a Hg monitoring programme is recommended for assessing the bioavailability of resuspended Hg from sediment.

Stripping voltammetric measurement of trace metal ions using screen-printed carbon and modified carbon paste electrodes on river water from the Eerste-Kuils River System.

Screen-printed carbon electrodes (SPCEs) and carbon paste electrodes (CPEs) were prepared as "mercury-free" electrochemical sensors for the determination of trace metal ions in aqueous solutions. SPCEs were coated with conducting polymer layers of either polyaniline (PANI), or polyaniline-poly(2,2'-dithiodianiline) (PANI-PDTDA). Furthermore, CPEs containing electroactive compounds with reactivity towards metal ions were employed to obtain enhanced selectivity. Optimised experimental conditions for Hg(2+), Pb(2+), Ni(2+) and Cd(2+) determination included the supporting electrolyte concentration, deposition potential (E(d)) and accumulation time (t(acc)). For the modified carbon paste sensors (MCPEs) it was found that -400 mV is an adequate deposition potential and an accumulation time of 120 s was adequate for the determination using the different constructed electrodes. Initial results showed linearity in the examined concentration range between 1 × 10(-9) M and 1 × 10(-6) M using the SPCE/PANI-PDTDA sensor on laboratory prepared standard solutions, while good selectivity for the different metal ions were obtained. Furthermore, the limit of detection (LOD) was determined for each of the sensors and for the SPCE/PANI-PDTDA sensor it was found to be 2.2 × 10(-13) M, while for the SPCE/PANI sensor the LOD was determined to be 8.4 × 10(-11) M. The MCPE sensors also showed good linearity between the concentration range of 1 × 10(-3) to 1 × 10(-9) M. The LOD values for the various MCPE sensors, were found to be Hg(II) - 1.3 × 10(-7) M; Cd(II) - 2.9 × 10(-7) M; Ni(II) - 3.2 × 10(-7) M; and Pb(II) - 1.7 × 10(-7) M for the CPE/PANI-PDTDA sensor. For the CPE/PANI sensor the LOD values were Hg(II) - 1.5 × 10(-5) M; Cd(II) - 8.6 × 10(-7) M; Ni(II) - 9.5 × 10(-7) M; and Pb(II) - 1.3 × 10(-6) M. For the CPE/MBT sensor the LOD values were Hg(II) - 3.8 × 10(-5) M; Cd(II) - 1.4 × 10(-6) M; Ni(II) - 1 × 10(-6) M; and Pb(II) - 6.3 × 10(-5) M. Very low detection was obtained for the SPCE/PANI-PDTDA sensor in Hg(2+) determination, while the MCPE sensors delivered sensitive simultaneous detection for Hg(2+), Pb(2+), Ni(2+) and Cd(2+) metal ions.

Mercury concentrations in water resources potentially impacted by coal-fired power stations and artisanal gold mining in Mpumalanga, South Africa.

Total mercury (TotHg) and methylmercury (MeHg) concentrations were determined in various environmental compartments collected from water resources of three Water Management Areas (WMAs) - viz. Olifants, Upper Vaal and Inkomati WMAs, potentially impacted by major anthropogenic mercury (Hg) sources (i.e coal-fired power stations and artisanal gold mining activities). Aqueous TotHg concentrations were found to be elevated above the global average (5.0 ng/L) in 38% of all aqueous samples, while aqueous MeHg concentrations ranged from below the detection limit (0.02 ng/L) to 2.73 +/- 0.10 ng/L. Total Hg concentrations in surface sediment (0-4 cm) ranged from 0.75 +/- 0.01 to 358.23 +/- 76.83 ng/g wet weight (ww). Methylmercury accounted for, on average, 24% of TotHg concentrations in sediment. Methylmercury concentrations were not correlated with TotHg concentrations or organic content in sediment. The concentration of MeHg in invertebrates and fish were highest in the Inkomati WMA and, furthermore, measured just below the US EPA guideline for MeHg in fish.

Acetylcholinesterase-polyaniline biosensor investigation of organophosphate pesticides in selected organic solvents.

The behavior of an amperometric organic-phase biosensor consisting of a gold electrode modified first with a mercaptobenzothiazole self-assembled monolayer, followed by electropolymerization of polyaniline in which acetylcholinesterase as enzyme was immobilized, has been developed and evaluated for organophosphorous pesticide detection. The voltammetric results have shown that the formal potential shifts anodically as the Au/MBT/PANI/AChE/PVAc thick-film biosensor responded to acetylthiocholine substrate addition under anaerobic conditions in selected organic solvent media containing 2% v/v 0.05 M phosphate buffer, 0.1 M KCl (pH 7.2) solution. Detection limits in the order of 0.147 ppb for diazinon and 0.172 ppb for fenthion in acetone-saline phosphate buffer solution, and 0.180 ppb for diazinon and 0.194 ppb for fenthion in ethanol-saline phosphate buffer solution has been achieved.