Valorized indigenous waste materials: potentially cheap and environmentally friendly adsorbents for the removal of toxic ions from wastewater

Abstract

The aim of this thesis was to identify an abundant, easily available, cheap and environmentally friendly waste material(s) in our locality, Botswana, that could be valorized (rejuvenated) by subjecting it to a simple green procedure, then apply as an adsorbent to clean wastewater through the removal or reduction of ions Co, Ni, Zn, Cu, Hg, Cr, Ag, Fe, Pb, Al, As, Cd, Sn, Ti, Mg, F-, Cl-, SO42-, PO43-, NO2-, BrO3-, and NO3- that usually exist in wastewater. After an extensive search, eggshells and fish scales proved to be the most readily available and abundant. These waste remains were independently pulverized to attain uniform sized particles with increased surface area to volume ratio needed for efficient adsorption of the targeted ions. The particle sizes of the pulverized waste powders estimated from both the Laval standard sieves and the scanning electron microscope micrographs were found to be ≤ 10 μm in each case, with spherical geometry and rough morphology, good features usually exhibited by good sorbents. The materials were further valorized by an optimal quantity (150 mL of vinegar for 80 mg of the valorized waste materials) of 12.7% vinegar for an optimal time (74 min). The procedure led to valorized waste materials (VWMs) with exposed functional groups on the surface of the waste powder particles that were attributed to the vinegar treatment and were thought to be the ones responsible for binding the targeted ions present in wastewater. Minitab 14 software was employed for the simultaneous modeling of the parameters of factors that affect sorption studies that included initial ion concentration, sorbent dose, contact time and solution pH which were found to be ≤ 28.44 mg/L, ≤ 84.29 mg/L, ≤ 88.63 min and ≤ 8.75 respectively. Employing the optimized conditions, the VWMs exhibited high percentage removal efficiencies toward removing the selected ions from raw wastewater samples averaging 92.13 % with %RSD < 2 for n = 3(triplicate). Fourier transform infrared spectrometer, X-ray diffractometer and Raman spectrometer spectra of the VWMs displayed multiple functional groups such as amines, carboxylic, hydroxyl, and carbonyls which are well known to bond well with ions through hydrogen and oxygen bonding, as well as hydroxyapatite and calcite known to be a good ion-exchanger. Furthermore, the calculations and assessment of the kinetic and thermodynamic parameters of the adsorption of the selected ions by the VWMs showed that the adsorption process fitted the Langmuir isotherm model (R2 ~ 1), fitted pseudo second order kinetics, was found to be spontaneous and endothermic respectively. All in all, the proposed VWMs with their high comparable percentage removal efficiencies, low cost (since they are collected from waste) and their environmental friendliness (since they are bio-materials thus bio-degradable) presented themselves as potential replacements of the expensive, non-environmentally friendly, commercial adsorbents (represented by charcoal in this thesis) that are commonly employed for the removal of toxic ions from wastewater in wastewater treatment plants.