Low-cost Rice Husk Ash and Silica for Chromium Ion Sorption from Aqueous System: Characterisation and Kinetics

Abstract

Several materials have been developed and used to remove Chromate (VI) from industrial wastewater before discharge into the environment because of its toxicity and lethality. In this study, low-cost adsorbents (rice husk ash (RHA) and silica (RHS)) were prepared from rice husks purchased locally from a rice mill industry and characterised. While the crystallinity and mineralogy of the adsorbents were analysed using powdered XRD crystallography, analysis of the physico-chemical properties was performed using standard procedures. Elemental analysis (CHN) was done using Perkin Elmer CHN elemental analyser and FTIR Spectrometer was used to determine the functional groups on the surface of the adsorbents at room temperature. The prepared samples were used as adsorbents for the removal of chromate (VI) ion in a batch sorption process with reaction conditions <i>vis–a-viz</i> adsorbent dosage = 0.5 g, adsorbate volume = 500 ml, adsorbate solution concentrations: 10-200 mg/l, time = 4 h, temperature = ambient temperature. The residual Cr (VI) concentration in the solution was analyzed spectrophotometrically at λ = 540 nm following the 1,5–diphenylcarbazide procedure. Results obtained for the characterisation of the adsorbents were similar and comparable with what obtained in literatures. Physico - chemical analysis revealed a high ash content of 89% and silica content of 95.83%. XRD analysis for the rice husk ash samples reflected the presence of cristobalite (SiO₂) in amorphous form and a characteristics broad peak at 2θ angle = 26.5°. FTIR analysis revealed the presence of silanol groups (Si-OH), silicic acid (Si-O-Si-OH) in the ash and Si-O-Si and Si-O) in the silica. The results of effect of time-concentration experiments of chromium ion sorption by the adsorbents revealed that Cr (VI) ion sorption was both time and initial solution concentration dependent with over 80% removal achieved within the first 30 min of contact for both adsorbents as the initial chromate concentration was increased. Kinetic modeling of the process showed up for pseudo second order, hinting on chemisorption as the mechanism of interaction.