Efficiency of Carica papaya Stem Activated with Phosphoric Acid and Sodium Hydroxide in Mining Wastewater Treatment

Abstract

AbstractChemically activated carbons prepared from the Carica papaya stem using phosphoric acid (CPSAC–H3PO4) and sodium hydroxide (CPSAC–NaOH) were used in sequestrating selected toxic metals (Cu2+, Mn2+, Co3+, Fe2+, Zn2+, Cd2+, Pb2+ and Cr3+) from mining wastewater. The mining wastewater was characterized for selected toxic metals using atomic absorption spectrophotometer (AAS). Batch adsorption studies were conducted using the CPSACs to remove selected toxic metals from the mining wastewater putting into consideration adsorbent dosage, agitation rate, contact time, pH and temperature parameters. The data obtained were fitted into isotherm (Freundlich, Langmuir, Dubinin–Radushkevich and Temkin) and kinetic (pseudo-first and pseudo-second orders, Elovich and intra-particle diffusion) and thermodynamic (standard enthalpy change, ΔH°; standard entropy change, ΔS°; and standard free energy change, ΔG°) models for its suitability. Three different error functions (sum of absolute errors—EABS, average relative errors—AREs and coefficient of determination—R2) were used for assessment of the linear and non-linear regression analyses of the selected adsorption isotherms and kinetic models. The removal efficiency of the selected toxic metals in CPSAC samples was between 92.97 and 100.00%. The adsorbent dosage, agitation rate, contact time, pH and temperature for optimum condition were 0.6 g (CPSAC–H3PO4) and 0.2 g (CPSAC–NaOH), 150 rpm, 60 min., 7 and 30 °C, respectively. Adsorption data followed the isotherms in the order of CPSAC–NaOH > CPSAC–H3PO4 and kinetics (i.e. pseudo-second order and intra-particle diffusion mechanism) in the order of CPSAC–H3PO4 > CPSAC–NaOH. Non-linear regression analyses using EAB, ARE and R2 error functions indicated to be better in analysing adsorption and kinetic models for the studied toxic metals. The ΔH°, ΔS° and ΔG° for the processes were 657.916, 64.288 and 22,012 kJ/mol, respectively. The adsorbents were efficient for removal of the selected toxic metals from the mining wastewater (though CPSAC–H3PO4 was most efficient); thus, it can be used for industrial application in wastewater treatment.KeywordsAdsorbentBatch adsorptionCarica papaya stemMining wastewaterToxic metals