Adsorption of zinc oxide nanoparticles using chemically modified sago waste under environmentally relevant conditions

Droepenu, Eric Kwabena (2021) Adsorption of zinc oxide nanoparticles using chemically modified sago waste under environmentally relevant conditions. PhD thesis, Universiti Malaysia Sarawak.

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Abstract

The increase global demand of sago starch which has subsequently increased the sago processing industries in Malaysia has inadvertently created its waste effect on the environment. Nevertheless, the amount of waste generated from this industry is also at the ascendency. However, these industries in Sarawak, Malaysia, are grappling with efficient ways of disposing these waste materials which are raising public concern. This study evaluated the potential of chemically modify sago hampas (acetylated, esterified and phosphorylated) as sorbent for removal of zinc oxide nanoparticles from water under environmentally relevant conditions through batch and column bed sorption study. Synthesized nanocrystalline zinc oxide and chemically modified sorbents were characterized using different techniques. Infrared spectroscopy of the treated sorbents saw the introduction of stretching and bending modes of carboxylic (C=O), Nitro (N=O) and phosphoric (P-OH & P=O) groups. The modification of the raw sago hampas enhanced the surface area from 1.18 m2/g to 29.02 m2/g in the case of acetylated carbonized sago hampas. The synthesized nanocrystalline zinc oxide were of spherical and flake-like morphologies with the particle and crystallite size in the range of 29-76 nm (Transmission Electron Microscopy) and 11-21 nm (X-Ray Diffraction) respectively. The environmental parameters of the UNIMAS lake determined were within range as per the Department of Environment, Malaysia and WHO water quality standards. The mean values recorded for pH, DO, TDS, EC, TSS, salinity, turbidity, BOD and COD were; 6.54-7.42, 3.21-7.35 mg/L, 42.7-49.8 mg/L, 84.5-99.5 µS/cm, 1.62-2.88 mg/L, 0.1-0.5 ppt, 5.12-6.84 NTU, 1.31-2.37 mg/L and 3.61-5.72 mg/L respectively. Results also showed that, the mean metal concentrations for Zn, Cu, Co, and Pb were found to be below the detection limit. However, the other metal ions follow the order Mg > Mn > Fe > Cr > Ni > Cd. Batch sorption study achieved equilibration at the 100th minutes, pH of 8 and sorbent mass of 2.0 g with the maximum percentage uptake of 90.5% (Acetylated Sago ‘Hampas’) as against 76.5% for the raw sorbent (Raw Sago ‘Hampas’). The equilibrium sorption process was well defined by Freundlich isotherm model as compared to Langmuir and Temkin on the basis of error analysis functions. However, the kinetic studies for the batch sorption showed pseudo-first order model as the best fitted for the process based on its lower values computed from the error analysis. An exothermic and non-spontaneous physical sorption process was realized in all the sorption studies according to the negative and positive values of the standard enthalpy change (ΔH°) and Gibbs free energy (ΔGo). However, the standard entropy change (∆S°) been positive demonstrated the occurrence of degree of freedom and strong affinity at the sorbent-sorbate interface. Experimental data from column sorption study established that, increase in influent concentration (1.51-5.04 mg/L) and flow rate (4.20-6.60 ml/min) decreased the breakthrough time between 150-30 min and 75-30 min respectively, whiles increase in column bed height (3-7 cm) increased the breakthrough time from 30-120 min averagely for the three sorbents. Comparative data analysis of Thomas, Adams-Bohart and Yoon-Nelson models using non-linear regression analysis tools (mean square error (RMSE), Chi square test (χ2) and coefficient of determination (1R2) showed that, the breakthrough data obtained for the sorption of Zn2+ ions onto the three sorbents was more adequately described by Adams Bohart model. Subsequently, Acetylated sago ‘hampas’ sorbent appeared to be the best sorbent for the sorption of Zn2+ ions both in batch and column sorption studies.

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