Functionalization of Synthesized NaP Zeolite with Silver Nanoparticles Capped with Alkyl Dimethyl Hydroxyethyl Ammonium for Sorption and Reduction of Chromate(VI) Oxoanions

A review on water treatment technologies for the management of oxoanions: prospects and challenges

Oxoanions are a class of contaminants that are easily released into the aquatic systems either through natural or anthropogenic activities. Depending on their oxidation states, they are highly mobile, resulting in the contamination of underground water. Above the permissible level in groundwater, they pose as threats to mammals when the contaminated water is consumed. Some of the health challenges caused are cancer, neurological, cardiac, gastrointestinal, and skin disorders. Several treatment technologies have been adopted over the years for the management of these oxoanions present in the aquatic systems. However interesting these treatment technologies might be, they also have their limitations such as cost-effectiveness, the complexity of the process, and generation of secondary pollutants. This work focused on some of the water treatment technologies applied for the removal of oxoanions. Some of the advantages and disadvantages of these treatment technologies are also highlighted. Amongst all the treatment technologies, adsorption is the most applied method for the removal of oxoanions. However, photocatalysis has a higher prospect since it is non-selective and secondary pollutants are not generated after the treatment process. Also, photocatalysis can simultaneously reduce and oxidise oxoanions as well as organic pollutants respectively.

Metal sorption studies biased by filtration of insoluble metal oxides and hydroxides

Toxic metals in the environment are often remediated using sorption techniques, particularly in aquatic and drinking water systems. However, a review of over 30 published sorption studies in the past two years alone revealed that the use of filtration to separate sorbed from unsorbed metals do not take into account metal hydroxide and oxide formation, and thus likely produce erroneous results. We quantified the effect of filtration on the removal of metal oxide/hydroxides from solution using a 0.45 μm filter as a function of pH, initial metal concentration and ionic strength for As, Be, Cd, Cu, Cr, Pb and Zn. We found that even when the initial metal concentration was as low as 0.1 mg/L, up to 93% of metals in solution were removed and up to 100% removal was observed when the initial metal concentration was 5 mg/L at a pH of 7. If this was unaccounted for, precipitated metal oxide/hydroxide removed via filtration will be inaccurately attributed to metal sorption. Additionally, we demonstrate that speciation modeling can underestimate the pH at which insoluble metal species form and therefore can only be used to approximate metal precipitation, especially in complex matrices. Overestimating the sorption capacity of sorbent materials has major implications if these sorbents are used for the purification of drinking water or other vital environmental remediation efforts. We recommend sorption studies using filtration prepare the appropriate matrix-matched control samples to quantify potential metal oxide/hydroxide formation.