Inability to Completely Remove Trace Contaminants from Drinking Water by Adsorption
Author(s): Wenfa Ng
Trace contaminant removal is of contemporary concern in drinking water and wastewater treatment, given increasing evidence of potential toxicity of various heavy metals and metalloids in water at parts per billion (ppb) concentration levels. Currently, only high-pressure reverse osmosis can reliably remove trace concentration contaminants from water but with high energy cost. Thus, the search is on for techniques and methods able to remove trace concentration contaminants with a lower energy footprint. Adsorption, a low energy technique for water and wastewater treatment is one possibility, which when coupled with the use of low-cost biomass as adsorbent, would further improve process efficiency. This synopsis describes work done on examining the use of local marine seaweed, Sargassum sp. for removing copper at low (4 to 20 ppm, parts per million) and trace concentration (< 1000 ppb), with and without ammonium ion interference at different solution pH. Specifically, to reduce organic leaching and improve mechanical stability, formaldehyde crosslinking was used and was effective in enhancing stability of seaweed between pH 3 and 9. But, residual organic leaching of ~ 4 ppm meant that modified seaweed was not suitable for drinking water treatment. Batch kinetic and equilibrium studies revealed that up to a threshold ammonium concentration of 50 ppm [NH4+-N], there was good sorption of copper on formaldehyde crosslinked seaweed (treated seaweed), with residual copper concentration of 0.5 ppm. Residual copper concentration increased with increase in ammonium concentration (up to 2500 ppm [NH4+-N]); thereby, highlighting a competitive binding effect for treated seaweed’s surface functional groups. Attempts to remove trace concentration copper highlighted feasibility of the approach. However, significant residual equilibrium concentration of copper remained in the solution. More importantly, thermodynamic considerations point to the existence of a finite residual equilibrium concentration for copper in water; thereby, making adsorption not feasible as a method for completely removing trace concentration copper. Additionally, surface concentration of copper at the adsorbent might, in some cases, be higher than the bulk solution; thus, leading to desorption of copper from the seaweed surface back to the solution. Collectively, adsorption is not capable of completely removing a contaminant such as is the case for reverse osmosis where the non-porous membrane serves as barrier separating treated water from raw influent.