Alkaline treated carob shells as sustainable biosorbent for clean recovery of heavy metals: Kinetics, equilibrium, ions interference and process optimisation

The aim of this work is to study the removal ability of cadmium(II) and cobalt(II) ions from aqueous solution by raw and chemicly treated carob shells as an ecofriendly and sustainable biosorbents. The biosorbents were characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis (CHNOS), scanning electron microscopy (SEM), Boehm titration and the point of zero charge (pHPZC). Biosorption experiments were carried out as function of solution pH, biosorbents dosage, contact time, initial metal ions concentration and temperature. The effect of some major inorganic ions including Na+, K+, Ca2+, Mg2+ and Al3+ on the biosorption efficiency was also investigated. From this studies it can be concluded that chemical pretreatment of the raw carob shells with NaOH strongly enhances its biosorption potential for the selected metals. Experimental results show that maximum biosorption capacity occurred at neutral pH medium. Kinetic data were best fitted with pseudo-first-order kinetic model instead of pseudo-second-order. Biosorption isotherms were best correlated with Langmuir model than with Freundlich model. The temperature doesn't have much influence on the biosorption yield. Based on this preliminary study, full factorial experimental design with two levels (24) was used for the optimisation of the process. Four independent variables, including solution pH, biosorbents dosage, initial metal concentration and treating agent were investigated. Thus, the optimal conditions for high biosorption of cadmium(II) and cobalt(II) were achieved at pH = 6, biosorbent dosage of 1 g/L, initial metal concentration of 50 mg/L for NaOH treated carob shells.