Instantaneous adsorption and simultaneous removal of complex dyes with different charges through bio-based sorbent

Abstract

One of the most challenging problems for wastewater treatment is its severe contamination with micropollutants such as dyes. Organic dyes and their challenges in the removal process together with their ability to transport coexisting pollutants are becoming a global threat to living organisms and the environment. Here, five types of organic dyes with different charges and sizes are sustainably removed from water using modified waste material as an economical and environmentally friendly sorbent. These dyes include methyl orange (MO), indigo carmine (IC), crystal violet (CV), methylene blue (MB), and malachite green (MG). Modified activated carbon derived from coconut shell (biochar) (AC-CS) was successfully prepared via a pyrolysis method using pre- and post-treatment modification to produce a sorbent with high porosity and rich functional groups. The structure, morphology, and porosity of the AC-CS were characterized through a series of techniques. The performance of AC-CS on the dye removal efficiency was studied through batch adsorption and column filtration experiments. Batch adsorption studies indicated removal efficiencies between 90 to 100% for all dyes by various adsorption mechanisms like electrostatic interactions (π–π stacking and H-bonding) and Fenton-like reaction. Furthermore, the AC-CS exhibited excellent removal efficiency for above mentioned dyes reaching the maximum adsorption capacities of 900 mg g⁻¹ (IC), 199 mg g⁻¹ (MB), 100 mg g⁻¹ (MG), 98 mg g⁻¹ (MO), and 39 mg g⁻¹ (CV). The adsorption isotherm and kinetics are in good match with pseudo-second-order models and thermodynamic studies showed dye molecule removal is spontaneous and thermodynamically favorable at higher temperatures. Also, our result for the simultaneous removal of a mixture containing quinary dyes revealed a higher removal performance at a shorter time than single dyes due to the effect of changing the surface charge of AC-CS by adsorbed dye molecules. Finally, the dynamics adsorption behavior of AC-CS was also studied by passing dyes and dye-mixture through a fixed-bed column representing this sorbent has potential for the treatment of micropollutant-containing wastewater in actual sewage and large-scale which provides basic support for the future practical applications.

Graphical abstract