Adsorption and desorption studies of Carica papaya stem activated with zinc chloride for mining wastewater treatment

The adsorption of eight selected potentially toxic metal ions from actual mining wastewater obtained from Igbeti, Nigeria, was established using activated carbon chemically prepared from Carica papaya stem with zinc chloride (CPSAC-ZnCl2) as activating agent. Characterization of the prepared CPSAC-ZnCl2 sample for surface morphology and functional groups was done by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy, respectively. An atomic absorption spectrophotometer (AAS) was utilized for characterization of the selected metals in the mining wastewater. Batch adsorption and desorption studies were conducted on removal of the metals from the sample using CPSAC-ZnCl2. The data obtained were fitted to isotherm (Freundlich and Langmuir); kinetic (pseudo-second-order and intra-particle diffusion) and thermodynamic (standard enthalpy change – ΔH°, entropy change – ΔS° and free energy change – ΔG°) models. These were considered under two error functions (sum of absolute errors – SAE,  coefficient of determination – R2) of linear and non-linear regression analyses. The SEM micrograph revealed that the CPSAC-ZnCl2 sample was 2.0–50.0 nm with FTIR spectra absorption peaks ranging from 746.2 to 3 987.0 cm-1. The initial concentrations of selected metals in the wastewater varied from 5.7 to 756.5 mg/L. The adsorbent dosage, agitation rate, contact time, pH and temperature for optimum condition of CPSAC-ZnCl2 were 0.6 g, 150.0 r/min, 60 min, pH of 7.0 and 30°C, respectively. The selected metals’ adsorption onto CPSAC-ZnCl2 followed Freundlich and Langmuir isotherm models pseudo-second-order kinetics with intra-particle diffusion mechanism. The ΔH°, ΔS° and ΔG° for the processes were 134.5, 64.5 and 22 012.0 kJ/mol, respectively. The adsorbent achieved an adsorption efficiency of above 95.0%, and is thus recommended for industrial application in remediating potentially toxic metals from wastewater.