Removal of chromium (VI) anions from water using peppermint leaf waste as an adsorbent

Hexavalent chromium [Cr(VI)] polluted water is highly toxic, penetrates cell membrane, causes DNA damage severe organ toxicity and carcinogenicity in humans and wildlife. Cr(VI) is among the most toxic substances and is classified as a group 1 human carcinogen. Peppermint ( Mentha x piperita L.) provides key ingredients for the production of herbal dietary supplements with documented health promoting properties. Approximately 5000 tons of processed peppermint leaves are discarded as non-edible waste into the environment. The strong aroma of dumped mint can attract rodents and insects, leading to localized pest infestations. This is concerning, given outbreaks of hantavirus originating from rodents and droppings and urine. The aim of this study was to convert waste peppermint leaves into adsorbents for the removal of Cr(VI) from wastewater. The untreated peppermint leaves (UPL) were converted to carbon (CPL), activating using H 2 SO 4 (APC) and characterized using SEM, FTIR and XRD. The activated carbon (APC) was evaluated for its ability to remove Cr(VI) through batch adsorption. SEM revealed a notably rough surface texture for UPL. After pyrolysis and activation with H 2 SO 4 , unique morphological properties such as voids and openings were observed in CPL and APC. Nitrogen adsorption-desorption studies indicated that APC exhibits mesopores. Batch adsorption experiments identified optimal conditions at a solution pH 3, an initial concentration of 100 mg/L, a temperature 313 K, and a contact time of 120 min. The data were consistent with the Langmuir model, which predicted a q max of 328.3 mg/g. Additionally, the data fitted the PSO model. Thermodynamic parameters showed that △H° was −6.142 kJ/mol; the negative value indicates that the uptake was an exothermic reaction. The negative ΔG° values indicate that the uptake on APC was spontaneous. SEM images after adsorption showed extensive cavities and openings on the surface of APC. FTIR results after adsorption showed that the intensity of the APC spectrum shifted significantly toward lower frequencies. Additionally, the peaks became less pronounced and shifted to higher wavelengths. These variations indicate the involvement of functional groups in the uptake of Cr(VI). The reusability of APC was tested up to seven cycles.