Effects of Melanin Extraction on Biosorption Behavior of Chestnut Shells Towards Methylene Blue

Abstract

Melanin is a kind of versatile biopolymer with strong binding capacities for cationic pollutants. The effects of melanin extraction from biomass on cationic pollutant adsorption of biomass remain unknown. Methylene blue (MB) was herein used as a model cationic pollutant, and chestnut shells were used as a melanin-containing byproduct of biomass. The MB biosorption behaviors onto the raw and melanin-extracted chestnut shells were comparatively studied. The biosorbents were contrastively characterized by BET surface area, scanning electron microscopy, energy-dispersive spectrometry, infrared spectroscopy, pH point of zero charges (pH pzc ), and cation exchange capacity (CEC). The MB adsorption capacity of each biomass was evaluated at different pH, contact time, initial MB concentration, and temperature. The melanin extraction resulted in higher values of adsorbent surface area, pH pzc , and CEC. It toned down the sensitivity of biomass to the solution pH, expanding the applicable pH range. It also introduced new types of binding sites or adsorption mechanisms. The biosorption process was well described by the chemisorption model of the pseudo-second-order equation and accelerated by the melanin extraction. The biosorption equilibrium data are well modeled by the Langmuir isotherm. The melanin extraction more than doubled the monolayer adsorption capacity. The improvement in the biosorption performance of chestnut shells by the melanin extraction is mainly owed to the chemical activation of binding sites and secondary to the physical modification. Melanin-containing biomass such as chestnut shells can be used in cascade for melanin production and subsequently for adsorptive removal of basic dyes such as MB.