Gitari, Wilson MugeraTutu, HlangananiMasindi, Vhahangwele2014-01-102014-01-102014-01-10Masindi, V. 2014. Adsorption of oxyanions of As, B, Cr, Mo and Se from coal fly ash leachates using A1/Fe modified bentonite clay. . . http://hdl.handle.net/11602/154http://hdl.handle.net/11602/154MENVSCDepartment of Ecology and Resource ManagementSouth African coal-fired power stations rely on the use of low grade bituminous coal for electricity generation. During coal combustion processes, this type of coal generates large volumes of waste materials which include Fly ash, bottom ash, boilers slag, flue gas desulphurization and un-captured particles. From the array of generated residues, coal fly ash constitute 90% and of this only 5% is beneficially used with the rest being disposed-off to land as ash dumps or slurried to ash retention ponds. On disposal, coal fly ash leaches out toxic chemical species on contact with the aqueous media hence posing hazardous effects to the aquatic and terrestrial environment. Of prime concern are Oxyanionic species such as As, B, Cr, Mo and Se. This study aims to investigate the adsorption of As, B, Cr, Mo and Se from coal fly ash leachates on Al3+/Fe3+modified bentonite clay. Coal fly ash samples used in this study were collected from Tutuka, ESKOM power plant in Mpumalanga and Bentonite clay samples were collected from ECCA (pty) Ltd. pH, Electrical Conductivity (EC) and Total Dissolved Solids (TDS) of the samples were determined using a CRISON multimeter probe. Modifications of bentonite clay with Al3+ and Fe3+ cations were done in batch procedures and the parameters optimized included contact time, adsorbent dose and adsorbate concentration. Cation Exchange Capacity (CEC), Point of Zero Charge (pHpzc) and Water Holding Capacity (WHC) of raw and modified bentonite clay were determined using standard methods. Surface areas of raw and modified bentonite clays were determined by BET. Elemental composition of raw and modified bentonite clay was determined by XRF and Mineralogical composition of raw and modified bentonite clay was determined by XRD. AAS and GFAAS were used to monitor AI3+and Fe3+concentration in the reaction mixture during and after modification while As, B, Cr, Mo, and Se were monitored using GFAAS, ICP- OES and JCP- MS. Adsorption of oxyanions of As, B, Cr, Mo and Se was carried out in batch experiments using synthetic solutions of the species. Effects of contact time, adsorbent dosage, adsorbate concentration and pH on the adsorption of As, B, Cr, Mo and Se were evaluated and optimized. The modification experiments revealed that loading of Al3+ onto bentonite clay interlayers is optimum at 60 minutes of contact time, 3 grams of adsorbent and 100 mg/L of adsorbate concentration and loading of Fe3+ onto bentonite clay matrices is optimum at 15 minutes of contact time, 2 grams of adsorbent and 100 mg/L of adsorbate. The CEC results showed that South African bentonite clay is characterized by high CEC of262 meq/l00g at pH 5.4 and 265.5 meq/l00g at pH 7.4 which decreased to 186.9 meq/l00g at pH 5.4 and 183.3 meq/l00g at pH 7.4 on introducing Al3+ onto bentonite clay interlayers and 195.5 meq/100g at pH 5.4 and 188.9 meq/100g at pH 7.4 on introducing Fe3+ onto bentonite clay interlayers. The results show that CEC is independent of pH. BET results indicated that the loading of Al3+ onto bentonite clay interlayers increased the surface area from 16 m2/g to 44.3 m2/g and 50 m2/g for Fe3+,external surface area from 11.1 m2/g to 33.1 m2/g for Al3+ and 37.4 m2/g for Fe3+,micro-pore area from 4.9 m2/g to 11.2 m2/g for AJ3+ and 12.5 m2/g for Fe3+. There was a slight difference in pHpzc of Al3+ modified bentonite clay (8.2) as compared to raw bentonite (8.0). For Fe3+ modified bentonite clay, the pHpzc was observed to increase from 8.0 for raw bentonite clay to 9.0. An increase in pHpzc shows that modification will favour adsorption of anions from aqueous media. WHC of bentonite clay was determined to be 42.7%. Elemental composition by XRF showed SiO2, AhO3, Fe2O3, CaO, Na2O, K2O and MgO as the major chemical species. XRF results also confirmed that bentonite clay is an aluminosilicate material. XRD disclosed that bentonite clay is dominated by montmorillonite as the major mineral phase. Characterization of fly ash leachates revealed that coal fly ash leachates contains 53 µg/L of As, 1730 µg/L of B, 271 µg/L of Cr, 165 µg/L of Mo and 170 µg/L of Se. Adsorption of As, B, Cr, Mo and Se onto Al3+ modified bentonite clay revealed that 30 minutes of contact time, 4 grams of adsorbent dosage, 10 mg/L of adsorbate concentration and pH 10 are the optimum conditions for removing those species from aqueous media. The adsorption affinity of those species obeyed the following order: B = Se > Mo = Cr > As. Adsorption of As, B, Cr, Mo and Se onto Fe3+ modified bentonite clay revealed that 30 minutes of contact time, 4 grams of adsorbent dosage, 10 mg/L of adsorbate concentration and pH 10 are the optimum conditions for removing those species from aqueous media. The adsorption affinity of those species obeyed the following order: B = Se > Mo > Cr = As. The adsorption capacities of modified bentonite clays were greater than the adsorption capacities of raw bentonite clay. The data fitted well to both Langmuir and Freundlich adsorption isotherm hence showing that the adsorption is energetically favourable. The unmodified clay showed poor adsorption for these chemical species. Both Al3+ and Fe3+ modified bentonite clay successfully removed As, B, Cr, Mo and Se from generated coal fly ash leachates to below DWAF water quality guidelines at the optimized conditions. This shows that AJ3+ or Fe3+ modified bentonite clay is an effective adsorbent for removal of oxyanionic species of As, B, Cr, Mo and Se from coal fly ash leachates and could be applied as a reactive barrier or liner in ash retention ponds.1 online resource (xxvi, 18 leaves)enUniversity of VendaAdsorptionUCTDOxyanionsCoalBentonite clayleachatesbentonite553.240968Fly ash -- South AfricaClay -- South AfricaCoal -- South AfricaBenotite -- South AfricaDissertationMasindi V. Adsorption of oxyanions of As, B, Cr, Mo and Se from coal fly ash leachates using A1/Fe modified bentonite clay. []. , 2014 [cited yyyy month dd]. Available from: http://hdl.handle.net/11602/154Masindi, V. (2014). <i>Adsorption of oxyanions of As, B, Cr, Mo and Se from coal fly ash leachates using A1/Fe modified bentonite clay</i>. (). . Retrieved from http://hdl.handle.net/11602/154Masindi, Vhahangwele. <i>"Adsorption of oxyanions of As, B, Cr, Mo and Se from coal fly ash leachates using A1/Fe modified bentonite clay."</i> ., , 2014. http://hdl.handle.net/11602/154TY - Dissertation AU - Masindi, Vhahangwele DA - 2014-01-10 DB - ResearchSpace DP - Univen KW - Adsorption KW - Oxyanions KW - Coal KW - Bentonite clay KW - leachates KW - bentonite LK - https://univendspace.univen.ac.za PY - 2014 T1 - Adsorption of oxyanions of As, B, Cr, Mo and Se from coal fly ash leachates using A1/Fe modified bentonite clay TI - Adsorption of oxyanions of As, B, Cr, Mo and Se from coal fly ash leachates using A1/Fe modified bentonite clay UR - http://hdl.handle.net/11602/154 ER -