A critical review of selected membrane- and powder-based adsorbents for water treatment: Sustainability and effectiveness

The increasing demand of sustainable water supply has overstretched the demand for newer technologies to purify water for drinking purposes. Adsorption technology, although ancient, has seen tremendous strides towards producing clean water through the introduction of effortless recovery of the adsorbents. However, this progress has some challenges. This review, therefore, looks into the progress and developments in adsorption technology, especially by comparing the popular powder/granular adsorbents with an emerging class of adsorbents which are based on polymeric membranes. Membrane-based adsorbents are easy to handle, and recovery is less cumbersome compared to powdered or granular adsorbents. The method of choice for the fabrication of these adsorbents is the melt blending method because it does not use any solvent, making it an environmentally friendly method. Considering the ease of melt blending and the abundant availability of suitable polymers, the membrane-based adsorbents would offer an environmentally friendly substitute to the available powder adsorbent. Membrane-based adsorbents are a relatively new class of adsorbents and research is still ongoing. Thus, a lot of research still needs to be done, including designing membrane-based adsorption systems compatible with industrial processes and developing suitable kinetic and equilibrium models. Another challenge is that the polymer matrix envelops some of the adsorption active sites and consequently reduces the surface area, which negatively affects the adsorption kinetics. This can be solved by using hydrophilic polymers; unfortunately, they are not compatible with the environmentally friendly melt blending. However, the durability of membrane-based adsorbents made from hydrophilic polymers is still in question, especially with the vigorous stirring required in adsorption. This review highlights an interesting research area that has not gained significant attention from the research community, namely tuning membrane-based adsorbents for comparative performance testing of these adsorbents relative to the more commonly used powdered or granular form of adsorbents.