3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
2 results
Search Results
Item Development of functionalised zeolite and bentonite for the recovery of platinum group elements (PGEs) and rare earth elements (REEs) from aqueous systems impacted by mining activities(2021) Mosai, Alseno KagisoThe economies of many countries, including South Africa, have been highly dependent on mining activities. Infrastructure including roads, buildings and power supplies has been improved over the years due to mining. As such, minerals such as gold, platinum group elements (PGEs), rare earth elements (REEs) and diamond continue to be mined, and their value increasing as demand outpaces supply due to diminishing resources. This has led to an increase in recycling efforts to salvage these precious elements. However, not much attention has been paid to deriving value from waste streams emanating from processing plants and discharged into the environment. To this end, the focus of this study was to use functionalised zeolite and bentonite as adsorbents for the recovery of PGEs and REEs from aqueous solutions of the type discharged from mineral processing plants. The research approach involved functionalising zeolite and bentonite with nitrogen containing ligands, namely: hydrazine, 3-aminopropyl(diethoxy)methylsilane (APDEMS) and spent yeast. These ligands were used because PGEs have been found to have strong interactions with nitrogen therefore, functional groups containing amine groups were ideal for functionalisation. Moreover, the stability constants between PGEs and nitrogen are very high (log k >10). The functionalised adsorbents were characterised using: Fourier transform infrared spectroscopy (FTIR) to determine functional groups; X-ray fluorescence (XRF) to determine the elemental composition; powder X-ray diffraction (PXRD) to determine the mineralogy; scanning electron microscope (SEM) to determine surface morphology; and elemental analyser to determine the extent of anchoring of the iv ligands. Textural properties (surface area, pore size and pore volume) were determined using Brunauer–Emmett–Teller (BET) for surface area analysis. The performance of the adsorbents in batch experiments was assessed by studying the effects of: pH; adsorbent dosage; initial concentration; contact time; and competing ions (Fe(III), Ca(II), Mg(II), K(I), Co(II), Ni(II), Hf(IV), Au(I), Zn(II) and other PGEs). Column studies were also conducted to assess the performance of the adsorbents under dynamic conditions. The effects of adsorbent bed height, pH, flow rate and initial concentration were investigated. Experimental results for both batch and column modes were assessed using various models. Adsorption capacity and efficiency models were used to determine the performance of adsorbents. Two-parameter (Langmuir, Freundlich, Dubinin-Radushkevich and Temkin) and three-parameter (Sips, Redlich-Peterson and Toth) isotherm models were used to determine the relationship between adsorbed elements and adsorbents. The adsorption mechanism was investigated using pseudo first-order, pseudo second-order, Elovich and intra-particle diffusion kinetic models. The performance of adsorbents in column mode was determined using column models such as Adams Bohart, Thomas, Yoon-Nelson, Clark, Wolborska, bed depth service time and modified dose response model. A generalised surface complexation model based on coupling parameter estimation (PEST) and the PHREEQC geochemical modelling code was used to obtain further insight into the adsorption mechanism. The model was calibrated using results from laboratory experiments. The findings showed that zeolite and bentonite in their natural forms had low adsorption efficiency (<5%) for PGE removal. However, after functionalising the materials with hydrazine, APDEMS or spent yeast, the uptake significantly increased (p <0.05), due to strong interactions between amine groups and the elements. Functionalisation of the v adsorbents was confirmed by FTIR and the elemental analyser. The surface area of bentonite was found to be higher than that of zeolite, resulting in the anchoring of more amine groups onto the former. The recovery of PGEs (Pt(IV), Pd(II) and Ir(III)) was highest at pH 2 and, significantly (p <0.05), decreased at pH >2, due to changing speciation and surface charge. However, the recovery of Rh(III) was efficient at pH ≥5 since, the Rh(III) species become positively charged with increasing pH, making them to be attracted to the negatively charged adsorbent surface. The adsorbents were highly efficient when concentrations of PGEs were low (2 mg L-1 ), with an adsorbent dosage of 10 g L-1 . The results also indicated that 90 min of contact was optimal for maximum adsorption of PGEs. The highest adsorption efficiency was >98, >98 and >95% for Pt(IV), Pd(II) and Ir(III), respectively, when functionalised bentonite was used and, >76, >91 and >60% for Pt(IV), Pd(II) and Ir(III), respectively, when functionalised zeolite was used. The uptake of the elements increased in the presence of competing ions, likely due to synergistic effects. The Langmuir isotherm model best fitted the adsorption data, indicating that the elements were only attracted to active sites with similar energies (i.e. - NH2 groups). The adsorption mechanism was described by the pseudo second-order kinetic model which is associated with adsorption via strong chemical interaction. The recovery of REEs was efficient when both the natural and functionalised adsorbents were used, but the natural adsorbents were used since their adsorption capacities were higher. The uptake of REEs was high (>98%) when bentonite was used at different solution conditions (pH> 2, concentration (0.5 – 10 mg L-1 ) and adsorbent dosage (5 – 50 mg L-1 )). However, the maximum uptake of REEs using zeolite was only achieved when the concentration was 2 mg L-1 or less, at pH >5 and adsorbent dosage of 10 g L-1 . The high uptake of REEs by natural zeolite and bentonite was attributed to the negative vi structural charge of the adsorbents over a wide range of pH, including acidic pH. Moreover, PHREEQC geochemical modelling code indicated that all REEs are positively charged even in solutions with pH >7. Thus, the adsorbents were always in favourable conditions for the adsorption of REEs. The high surface areas of the adsorbents also played a major role on the uptake of the elements. In contrast, the functionalised zeolite and bentonite had lower surface areas compared to their natural counterparts and the uptake was low at highly acidic pH solutions since the structural surface charges of the adsorbents were positive. The adsorption of most REEs (e.g. Y, Pr) onto bentonite was described by the Langmuir isotherm model and some (e.g. Ce, Gd, Tm) by the Freundlich model. Thus, most of the REEs adsorbed onto active sites with similar energies but some onto different active sites with different energies. The adsorption of all REEs onto zeolite was described by the Langmuir isotherm. Adsorption mechanism of REEs was described by the pseudo second-order kinetic model. Breakthrough curves from column studies indicated that the uptake of PGEs and REEs increases with increasing bed height due to a higher number of binding sites. Higher flow rates (>2 mL min-1 ) do not give the elements enough time (residence time) to interact with the adsorbents therefore, breakthrough times were observed earlier than when lower flow rates were used. Agreement of experimental and modelled data (p >0.05) indicated that the coupling of PHREEQC to PEST can be used to determine the efficiency of the adsorbents on the recovery of precious elements when necessary conditions such as pH, concentration and surface area are determined and used for the calibration of the PHREEQC model. Also, it was observed that only a few experiments were required for this purpose. vii Indicative cost-benefit analyses showed that the efficient uptake of PGEs and REEs by amine-functionalised and natural adsorbents, respectively, implied that these elements could be potentially recovered from wastewaters. For instance, up to 25 ounces of Pt per kg of yeast-functionalised bentonite and 54 ounces of REEs per kg of natural bentonite could be recovered, which are higher recoveries compared to amounts cited for economical mining. This could be important even for low level concentrations in waste streams as companies seek to salvage these metals from sundry sources to make up for increasing costs, declining natural resources and increasing demand. The successful coupling of PHREEQC and PEST suggests that limited experimental data can potentially be used to do predictions of sorption processes in generalised surfaces such as those studied here. Upon successful calibration, complex reactions and time consuming experiments can be simulated. This procedure can potentially be used by mining companies and other industries as a preliminary step to determine the feasibility of administering adsorbents to recover elements from wastewaters. The use of PHREEQC coupled to PEST for simultaneous recovery of REEs using a generalised surface could not be done but, would be important for future studies. Such studies should also focus on the recovery of elements in the presence of organic compounds such as low molecular weight humic and fulvic acids.Item Modelling of sorption of trace elements in an agricultural soil impacted by mining activities(2017) Mosai, Alseno KagisoThe development of the economy of South Africa and many other countries has been highly dependent on mining industries. Minerals such as gold, platinum, diamond and many others have been mined and continue to be mined. Despite the importance of these minerals, their processing comes with social and environmental problems. During the processing of these minerals, trace elements such as copper, chromium, nickel, mercury, uranium, molybdenum and many others are released as wastes into the environment either, directly or indirectly. The release of the elements into the soil is of concern due to the possibility of groundwater system contamination. The presence of these elements in the groundwater system poses serious challenges to the wellbeing of life forms, due to their toxicity, when they exceed threshold limits. From the processing plants, these elements could be released onto the soil, and mobilise to groundwater, increasing the already existing environmental crisis due to water pollution. Once these elements are in the water, access to living organisms becomes easier through the food chain. Some of these elements are not biodegradable and thus persist in the environment as well as in the bodies of living organisms. They can cause serious health problems because of their toxicity effect. In humans, these elements can be carcinogenic, and also cause chronic disorders, kidney failures, defects in infants, bone and vascular diseases which could also be lethal. It is therefore of importance that these elements are neither bioavailable nor bioaccessible to living organisms. When these elements are mobile in the soil, the probability of reaching groundwater increases. Water, an important natural resource should always be protected from such pollutants. The demand for unpolluted water has been rising every year in the world due to increasing population, extended droughts and improper disposal. This research was dedicated to determining the behaviour of elements in an agricultural soil impacted by mining activities. Agricultural soils are sometimes exposed to pollutants that could originate from dust fallout or precipitation; fertilisers and manure; pesticides; and water used for irrigation. Understanding the iv processes that control the distribution of these pollutants in agricultural soils is an important risk assessment measure, considering that such pollutants have the potential of being taken up by crops and vegetables or transported to groundwater. In this study, a soil on a farm that grows vegetables for commercial purpose. Cabbage, spinach, carrots and potatoes are some of the vegetables grown on the plot and sold to markets in Pretoria and Johannesburg. The plot is in the vicinity of smelting operations in the North West Province. The mobility of trace elements in the soil can be controlled, depending on the type and properties of soil. Hence in this research, the ability of the soil to adsorb elements entering the soil is studied. The batch experimental work was performed to determine the effect of pH, initial concentration (5 - 100 mg/L), competing ions (Fe3+, Ca2+, Co2+, Mg2+, K+, Ni2+ and Zn2+), fertilisers (ammonium nitrate, ammonium phosphate and calcium chloride) and plant exudates (acetic acid, citric acid and oxalic acid as well as ethylenediaminetetraacetic acid (EDTA) which is often used as proxy organic ligand (found in manure)) on the adsorption of cadmium (Cd), copper (Cu) and chromium (Cr) onto an agricultural soil. The PHREEQC geochemical modelling code was used to complement experimental methods in predicting processes and to further assess the leaching behaviour of the elements. Powder X-ray diffraction (PXRD) and X-ray fluorescence (XRF) were used to determine the mineralization of the soil. The structural features of the soil were determined using Fourier Transform Infrared spectroscopy (FTIR) and the element content was determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The point of zero charge (PZC) of the soil was found to be 8.3 and the cation exchange capacity (CEC) of 51.6 meq/ 100g. In the absence of fertilisers and plant exudates, the soil exhibited a similar high adsorption for elements at all initial concentrations by all the elements. Most (> 90%) of the elements were adsorbed within the first 3 minutes of contact with the soil. Langmuir, Freundlich and Dubinin-Radushkevich adsorption isotherms were used to describe the experimental data for the elements. Kinetic rates were modelled using pseudo first-order and pseudo second-order equations. Pseudo v second-order gave the best fit for all the elements (R2 >0.999) indicating chemisorption. The effect of pH on Cd and Cu was insignificant however, the adsorption of Cr decreased with pH. The presence of competing ions decreased the adsorption of cadmium more than that of the other analyte elements. The soil was generally effective in adsorbing and retaining the elements. However, the retention was highly dependent on elemental speciation and prevailing conditions e.g. pH (as in the case of Cu and Cr). Such changes in conditions would have implications for groundwater quality. The effect of plant exudates and EDTA was studied and the results showed that low molecular weight organic acids (LMWOAs) viz acetic acid (AA), citric acid (CA) and oxalic acid (OA) and EDTA significantly (p < 0.05) decreased the adsorption capacity of the elements onto the agricultural soil. AA had the least effect on the adsorption capacity of the elements whereas OA and EDTA strongly prevented the adsorption of the elements. Moreover, some of the elements which were already in the soil including those which were not under study such as Ca and Mg were desorbed from the soil by OA and EDTA. Thus, the mobility of the elements was increased by the presence of plant exudates, increasing groundwater contamination and consequently threatening the health of living organisms. Agrochemicals such as fertilisers, stabilizers and pesticides are constantly applied to agricultural soils to improve the fertility of the soil for better crop production however; their presence may affect the mobility and bioavailability of elements in the soil. The effect of ammonium nitrate and ammonium phosphate as well as calcium chloride on the adsorption of Cd, Cu and Cr onto an agricultural soil was studied. The effects of initial concentrations of the elements (5 – 50 mg/L), concentrations of fertilisers (0.01 – 0.1 mol/L) and pH (3 - 8) on the adsorption of Cd, Cu and Cr were studied. The initial concentration of the elements and the concentration of fertilisers had no significant effect (p > 0.05) on the adsorption capacities of Cu and Cr at pH 5. But, ammonium nitrate and calcium chloride decreased the adsorption capacity of Cd. The adsorption of Cd onto the soil was reduced as the concentration of fertilisers increased. The adsorption of Cd was lower than that of Cu and Cr at all pH values. The agricultural soil was found to vi be an effective adsorbent in preventing the mobility of Cu and Cr in the presence of fertilisers but not for Cd whose adsorption was significantly affected by the presence of ammonium nitrate and calcium chloride. A continuous flow fixed-bed column script with specified conditions simulating the natural environment was utilised in PHREEQC for column studies. The geochemical computer model PHREEQC can simulate solute transport in soil surfaces. The effect of initial concentration (100 and 300 mg/L) of the elements, column bed depth (5 and 10 cm) and pH (3, 5, 7 and 10) were considered in this study. The adsorption capacity was affected by initial concentration of the elements since the breakthrough curves at higher analyte concentrations were reached at lower pore volumes than at low concentrations. This can be attributed to the fast occupation of active sites of the soil at higher concentrations. The results from PHREEQC indicated that the conditions used would lead to the oxidation of Cr3+ to Cr6+ leading to the formation of HCrO4- and Cr2O72- which were not favoured for adsorption by soil surfaces due to high solubility. This could have potential implications on the quality of groundwater in regions with similar conditions. Thus, the leaching of Cr6+ onto the agricultural soil will be high in areas where remediation techniques are not applied. The changing of bed depth from 5 to 10 cm did not have an effect on the adsorption of the elements. The ability of the soil surfaces to adsorb Cd and Cu even at lower bed depth implies that the soil will be effective in preventing the leaching of the elements to groundwater due to strong surface interactions of the elements with the soil. The results from PHREEQC showed that the adsorption of Cd and Cr onto the soil surface was not affected by pH. The results for Cr were contradicting with those obtained from laboratory experiments which could be due to the conditions used in PHREEQC. The change in the speciation of Cu at basic conditions decreased the ability of Cu adsorption onto the soil surfaces. The Cu2+ was converted to Cu(OH)2 which were large in size and thus only a small amount could be adsorbed since the other adsorption sites were covered by the large species. This research had notable outputs in the form of publications which will form an important repository of information.