Evaluation of thermodynamics and effect of chemical treatments on sorption potential of Citrus waste biomass for removal of anionic dyes from aqueous solutions

The intention of this study is to assess the sorption potential of Citrus waste biomasses, i.e. Citrus reticulata, Citrus sinensis, Citrus limetta and Citrus paradisi having optimum sorption capacity for anionic reactive dyes. Citrus sinensis biosorbent showing maximum sorption capacity was selected (qe 13.99, 15.21, 14.80 and 27.41 mg/g for Reactive yellow 42, Reactive red 45, Reactive blue 19 and Reactive blue 49, respectively). The effect of chemical treatments to augment the sorption capability was evaluated and pretreatments of biosorbent were carried out with a range of organic and inorganic reagents. Acetic acid treated biosorbent showed enhanced sorption potential for elimination of Reactive yellow 42 (qe 17.64), Reactive blue 19 (qe 23.31), and Reactive blue 49 (qe 33.53) whereas acetonitrile resulted in improved sorption capacity for Reactive red 45 (qe 18.18). Sulphuric acid, phosphoric acid, ethanol, methanol, EDTA and sodium chloride treated biosorbent showed decreased sorption capacity for each reactive dye. Thermodynamic parameters like ΔG°, ΔH° and ΔS° were determined and negative enthalpy values; −35.99 kJ/mol (Reactive yellow 42), −25.46 kJ/mol (Reactive red 45), −8.99 kJ/mol (Reactive blue 19), and −14.17 kJ/mol (Reactive blue 49) established the exothermic nature of process. The sorption was accompanied by a decrease in entropy as demonstrated by negative entropy values of −0.12, −0.08, −0.04 and −0.04 kJ/mol K for Reactive yellow 42, Reactive red 45, Reactive blue 19 and Reactive blue 49, respectively. Scanning electron microscopic images of biosorbent surface revealed the porous nature of biosorbent surface.