The synthesis, characterization and performance evaluation of polyphelenediamine- and polypyrrole- clay composites for removal of oxo-anionic wastewater contaminants

• Main Author: Mdlalose, Lindani Mbalenhle
• Format: Others
• Language: en
• Published: 2018
• Online Access: https://hdl.handle.net/10539/25862

A thesis submitted to the Faculty of Science, University of the Witwatersrand in fulfilment of the requirements for the degree of Doctor of Philosophy 2018 === Contamination of water bodies by numerous pollutants is a worldwide problem that endangers the environment and health of human beings, animals and aquatic life. Hexavalent chromium (Cr(VI)) for example is used in different metal products and processes which makes it a common environmental contaminant. Because of its high mobility in aqueous phase, and improper storage or unsafe disposal practices, leakage of Cr(VI) into water streams and ground water is a common occurrence. While Cr(III) is an essential micronutrient, Cr(VI), however is highly toxic posing serious health risks. Additionally, phosphorus is a limiting nutrient for the growth of organisms in most ecosystems, but, excessive discharge of phosphate ions in water systems leads to profuse algal growth, and is detrimental to both the environment and the ecosystem. This research focused on the development of suitable functional adsorbents for the removal of Cr(VI) complexes and phosphate ions from wastewater. Poly(para-phenylene)- (PpPD) and polypyrrole-based composites were synthesized through chemical oxidation polymerization, and investigated for Cr(VI) remediation. Poly(phenylenediamine) isomers were synthesized through different chemical oxidation methods for the uptake of phosphate ions in wastewater. Transition metals modified bentonite clay adsorbents were developed to remove phosphate ions in aqueous solution.The adsorbents were characterized using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD), Brunauer-Emmett-Teller (BET), Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Thermogravimetric analyzer (TGA) and X-ray photoelectron spectroscopy (XPS) instruments. Adsorption kinetics and isotherm models were investigated. In the first study of this work (paper I), PpPD and PpPD-clay composite were successfully prepared and applied for Cr(VI) removal and reduction in aqueous solution. Characterization by XRD demonstrated that PpPD molecules intercalated into clay galleries. Additionally, PpPD functional groups dominated in the composite even though the signal bands were smaller than the pristine polymer bands indicating that there was a formation of polymeric structure inside the organoclay interlayer spaces. Batch adsorption studies showed that pH, adsorbent dosage, contact time and Cr(VI) concentration affected the degree of adsorption. The Langmuir maximum adsorption capacity for Cr(VI) was 217.4 mg/g and 185.2 mg/g whereas for total Cr it was 193.3 mg/g and 148.8 mg/g for PpPD and PpPD-organoclay, respectively at an optimum pH of 2. Paper II focused on the chemistry of Cr(VI) adsorption by PpPD and adsorbent regeneration. The adsorption mechanism on the material surface was revealed by XPS and FT-IR. Cr(VI) was reduced to Cr(III) which complexed onto the adsorbent surface at the studied pH of 2 and 8. Desorption of the adsorbed Cr was conducted using NaOH (0.05 M) and HCl (0.1 M). The PpPD adsorbent performed optimally for eight cycles and still retained about 80% adsorption efficiency at the 10th cycle using an initial Cr(VI) concentration of 100 mg/L. Treatment with the regenerants showed irreversible oxidation reaction for the adsorbents while still removing Cr(VI) for several cycles. To investigate the toxicological impact on seed germination due to contact with used adsorbents, phytotoxicity test was investigated. Seed germination severely diminished to 35% and 14% (respective to control) in the presence of P-p-PD-MMT and P-p-PD. In paper III polypyrrole-clay composite was synthesized and also proved to be an effective adsorbent for Cr(VI) removal. A percentage Cr(VI) removal of 99% was obtained at pH 2 using adsorbent dosage of 0.15g for 100 mg/L Cr(VI) concentration for 3h in a batch mode. Due to its excellent adsorption properties, the composite was regenerated using different varied concentrations of eluents (NaOH, NH4OH, HCl, NH4Cl and HNO3). Desorption and regeneration using 0.01 M NaOH and 0.5 M HCl gave more regeneration cycles where the first 5 regeneration efficiencies were still greater than 80%. EDX determined the elemental components of the polypyrrole-clay composite before and after Cr(VI) adsorption. It demonstrated a significant decrease of Cl- ions after adsorption which is attributed to ion exchange mechanism between Cl- ions and Cr(VI) during the adsorption process. Investigation of the adsorption behaviour revealed a decrease in thermal stability of the composite after several adsorption cycles while treating the adsorbent with the regenerants as a result of material oxidation and deterioration due to Cr(VI) exposure in acidic medium and the impact of the regenerants. According to FT-IR analysis, polypyrrole-clay bands shifted to a higher wavenumber after Cr(VI) adsorption due to the change in skeletal vibrations as a result of Cr(VI) species adsorbed onto its surface Paper (IV) described synthesized adsorbents for phosphate removal. The study presented the development and performance of two sets of poly(phenyelenediamine) (PPD) isomers synthesized from ammonium persulphate ((NH4)2S2O8) and potassium dichromate (K2Cr2O7) as oxidants. The chemical structure of the adsorbents were determined using FT-IR, TGA and XRD. Amorphous morphology dominated in all the polymers with poly(m-phenylenediamine) PmPD being more amorphous and PpPD was the least. Batch adsorption studies showed improved adsorption capacity for K2Cr2O7 synthesized polymers. K2Cr2O7 oxidant played a major role in providing trivalent chromium metal which improved the phosphate uptake. This is attributed to the Lewis acid-base interaction where trivalent chromium acts as an acid and phosphate ions serve as a base. Batch adsorption results showed that solution pH, contact time and initial concentration influenced phosphate adsorption with the maximum adsorption capacities of 143 mg/g, 217 mg/g and 69.0 mg/l for PoPD, PmPD and and PpPD adsorbents, respectively. Adsorption reached equilibrium at about 300 min at an optimum pH of 2.0. The adsorption isotherms were described by Langmuir isotherm and the kinetic data were described better by pseudo-second order kinetic rate model implying adsorption onto homogeneous surfaces and the mechanism of adsorption was attributed to chemisorption. Desorption was conducted on the meta substituted PPD using NaOH (0.05 M) which displayed effective desorption capacity and exhibited commendable adsorption for re-use. The adsorbent also proved to be selective to phosphate ions at the background of much higher concentrations of sulphate and nitrate anions due to the presence of Coulombic and Lewis-acid-base interactions. In paper (V), remediation of phosphate ions was examined using modified bentonite clay as an adsorbent. Modification was achieved by incorporating Fe, Ni and Co metal salts using precipitation method. Adsorbents were characterized by FT-IR and XRD. The results showed significant amorphosity for metal modified bentonite compared to the parent bentonite. The adsorption capacity for all studied bentonite-based materials increased with increasing initial phosphate concentration and adsorption mechanisms were influenced by the solution pH. The maximum adsorption capacity of 6.57 mg/g, 20.88 mg/g, 29.07 mg/g and 46.95 mg/g were obtained for Bent, Fe-Bent, Ni-Bent and Co-Bent, respectively. The adsorption rate fitted pseudo-second order for all adsorbents. Langmuir isotherm model described the phosphates removal for all adsorbents at an optimum pH of 3. === MT 2018