Recent innovative research on chromium (VI) adsorption mechanism

Abstract

Chromium (VI) is an important heavy metal commonly found in tannery wastewater. In order to manage this type of environmental pollution, there needs to be a better understanding of Cr(VI) adsorption mechanisms. This article reviews the recent usage of various adsorbents to adsorb Cr(VI) from wastewater. The paper provides an overview of the removal capacity of numerous adsorbents in terms of Cr(VI) speciation, empirical modeling, surface complexation model (SCM) and spectroscopic measurements carried out by a wide number of researchers. Special emphasis was given to summarise various proposed Cr(VI) adsorption mechanism. It has been noted that Cr(VI) adsorption has been studied using a wide range of adsorbent materials with adsorption capacity from low to higher depending several factors including the nature and type of adsorbent, surface property of adsorbent, surface modification, pre-treatment of substrate, coexisting substrate and experimental conditions (equilibration time, solution pH, initial Cr(VI) concentration, ionic strength). Cr(VI) adsorption was highly dependent on solution pH and Cr(VI) adsorbed strongly in acidic pH while it exhibited weak or no adsorption in alkaline pH condition. Cr(VI) adsorption data has been studied using proton stoichiometry, kinetic, Langmuir, Freundlich, and surface complexation model (SCM). Adsorption data commonly fitted with a pseudo-second-order (PS2) kinetic model and Langmuir equation. Cr(VI) adsorbed typically by inner-sphere Cr(VI) complex at pH below 6.0 and by outer-sphere Cr(VI) complex above pH 6.0. Proton stoichiometry indicated that more than one reaction was involved in Cr(VI) adsorption. This review can be a centrepiece of researchers who wish to work on Cr(VI) adsorption. This study will shed new light on the understanding Cr(VI) adsorption onto the substrate at the water-solid interface and develop high-performance substrate to remove pollutants efficiently from wastewater.