Preparation and characterization of steam-activated and green alkali-activated carbon from coconut shells

Abstract

This study adopts the ongoing green approach to develop activated carbon from natural, eco-friendly materials offering a sustainable alternative to commercially available products. The environmental problem that arises from consistent discharge of KOH during activation can be alleviated by adopting the green synthetic route of production. This study presents the extraction of alkali from cocoa pod husks (CPHs) as less harmful alternative to toxic commercial KOH. The extracted alkali was further utilized in activation of carbon to produce novel coconut shell alkali activated carbon (CSAA). Simultaneously, coconut shell steam activated carbon (CSSA) was prepared using steam activation technique in a stainless-steel reactor at a flow rate of 120 mL/hour at 800° °C for 5 h. Both coconut shell based activated samples (CSAA and CSSA) were characterized using Fourier Transform Infrared (FTIR) analysis for surface functional groups identification. Their surface morphologies and elemental composition were investigated using Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX). X-ray Diffraction analysis (XRD) were employed for textural analysis while Brunauer-Emmett-Teller (BET) methods examined their specific surface areas as well as pore volumes. The yield of carbon from 100 g of pre-treated precursor was estimated as 89.25 % before activation. The concentration of the extracted alkali was evaluated using the titration technique. The pH of 11.6 obtained for extracted alkali suggests alkalinity which was confirmed by observable colour change of solution to yellow on the addition of drops of methyl orange indicator. The EDX result confirms that extract is composed primarily of metals with potassium having the highest value of 53.60 %wt. The FTIR results of the activated carbons showed the presence of -OH, C-H, and C-O, C O functional groups suggesting carbonization and activation was successfully conducted. Additional peaks observed for CSAA can be attributed to the presence of K-O bonds. Both activated carbons are characterized by large BET surface areas of 1200.20 m²/g for CSSA and 1250.20 m²/g for CSAA. These large surface areas invariably make them suitable adsorbents for wastewater remediation, water purification, and trapping of gases. The higher pHZC of 8.0 obtained for novel CSAA also confirms the presence of surface basic functional groups which can be attributed to successful alkali activation. Based on these analytical findings, adoption of green extracted alkali in activation of carbon is a safe alternative to the use of chemical activators. The entire procedure from extraction to activation is clean without the release of any toxic substances to the environment. • Coconut shells were used as a natural, local source for activated carbon production. • Alkali was green-extracted from cocoa pod husks and chemically characterized. • Extracted alkali activated carbonized coconut shells to form novel CSAA. • Steam activation was applied to prepare coconut shell steam-activated carbon (CSSA). • CSAA and CSSA were characterized using FTIR, SEM-EDX, XRD, BET, and pHpzc. • CSAA exhibited a high BET surface area (1250.20 m 2 /g) and pore volume (0.61 cm 3 /g).