Caliphs M. Zvinowanda

A novel adsorbent for heavy metal remediation in aqueous environments

The objective of this study was to investigate the possibility of using maize tassel as an alternative adsorbent for the removal of chromium (VI) and cadmium (II) ions from aqueous solutions. The effect of pH, solution temperature, contact time, initial metal ion concentration and adsorbent dose on the adsorption of chromium (VI) and cadmium (II) by tassel was investigated using batch methods. Adsorption for both chromium (VI) and cadmium (II) was found to be highly pH dependent compared to the other parameters investigated. Obtained results gave an adsorption capacity of 79.1 % for chromium (VI) at pH 2, exposure time of 1h at 25 °C. Maximum capacity of cadmium of 88 % was obtained in the pH range of 5-6 at 25 °C after exposure time of 1 h. The adsorption capacities of tassel for both chromium (VI) and cadmium (II) were found to be comparable to those of other commercial adsorbents currently in use for the removal of heavy metals from aqueous wastes. These results have demonstrated the immense potential of maize tassel as an alternative adsorbent for toxic metal ions remediation in polluted water and wastewater.

Application of maize tassel for the removal of Pb, Se, Sr, U and V from borehole water contaminated with mine wastewater in the presence of alkaline metals.

In this study, the removal of Pb(II) from aqueous solutions by tassel powder was studied and optimised. Batch experiments were conducted on simulated solutions using tassel powder adsorbent and the effects of contact time, pH and concentration on the extent of Pb (II) removal was studied. Equilibrium and kinetic models for Pb(II) sorption were developed by considering the effect of contact time and concentration at optimum pH 4 and fixed temperature(25 degrees C). The Freundlich model was found to describe the sorption energetics of Pb(II) on tassel more fully than the Langmuir. A maximum Pb(II) loading capacity of 333.3mg/g on tassel was obtained. The adsorption process could be well described by both the Langmuir and Freundlich isotherms with R(2) values of 0.957 and 0.972, respectively. The kinetic parameters were obtained by fitting data from the effect of contact time on adsorption capacity into the pseudo-first, pseudo-second-order and intra-particle diffusion equations. The kinetics of Pb(II) on tassel surface was well defined using linearity coefficients (R(2)) by pseudo-second-order (0.999), followed by pseudo-first-order (0.795) and lastly intra-particle diffusion (0.6056), respectively. The developed method was then applied to environmental samples taken from borehole waters contaminated with mine wastewater. The removal of Pb (ND-100%), Se (100%), Sr (5.41-59.0%), U (100%) and V (46.1-100%) was attained using tassel. The uptake of the metals from environmental samples was dependent on pH, ionic strength and levels of other competing species.

Improved derivatisation methods for the determination of free cyanide and cyanate in mine effluent

Generally, the level of cyanide in waste effluents is too high to be discharged into the environment. Consequently, treatment regimes are necessary in order to protect the environment. However, the cost of most of the treatment methods is expensive and not sensitive enough and, therefore, cannot always be justified. In this research, cyanide speciation products, free cyanide (CN(-)) and cyanate (CNO(-)) were determined by highly sensitive derivatisation methods followed by spectrometric analysis. Spectral scans were carried out for pure and environmental samples derivatives in order to evaluate the possibility of interfering species. For CN(-) a linear range from 0.01 to 80.0mg/L was determined. In the case of CNO(-), the linear range was between 0.02 and 80.0mg/L. The detection limits were 0.05 and 0.20mg/L for CN(-) and CNO(-), respectively. These values are in good agreement with those reported in literature. The concentration ranges of the speciation products in environmental samples were 0.70-52.0mg/L and 0.50-76.0mg/L for CN(-) and CNO(-), respectively. These values were well above their acute toxicity levels. Increase in cyanate levels in the effluent with time was clearly observed while the concentration of cyanide decreased. This was attributed to the oxidation of CN(-) to CNO(-).

Fixed bed Column Adsorption of Cu (II) onto Maize Tassel-PVA Beads

The intention of this study was to explore the efficacy and feasibility for Cu (II) adsorption onto fixed bed column of maize tassel-PVA beads. The effects of flow rate and bed height were explored. The Thomas, Adams and Bohart and Yoon-Nelson models were analysed to evaluate the column adsorption performance. The adsorption rate constant and correlation coefficient associated to each model for column adsorption was calculated. Thomas model indicated that increase in column height and flow rate increased the values of k Th and q o . Thereafter, the value of q o decreased with decreasing bed height. With Adam-Bohart model, the values of kAB increased as the flow rate and the bed height increased and continued to increase with decrease in bed height. However, the value of N o first increased with increase in flow rate and bed height, but later decreased with decrease in bed height. This suggests that the overall system kinetics may have been influenced by external mass transfer, particularly in the initial part of adsorption in the column. In the case of Yoon-Nelson model, the rate constant k YN and τ increased with increase in bed height and flow rate. However, decrease in bed height at a flow rate of 2.33 × 10 -3 L min -1 resulted in an increase in τ while the values of k YN decreased. The three models gave high values of R2 (0.9694-0.98920), although the Adam-Bohart model values were, on average, the least. Therefore, it can be said that both the Thomas and Yoon-Nelson models describe the behavior of the adsorption of Cu (II) in a fixed-bed column better than Adam-Bohart.

Process optimization of freeze desalination of brine using HybridICETM pilot plant

Freeze desalination of synthetic brines prepared from sodium chloride was investigated. Brine solutions of levels between 2.3% to 10% sodium chloride were used as feed to the plant operated in a batch and continuous mode. The effects of pump, gear motor speed, and differential temperatures on ice formation and purity were investigated. The HybridICE process produced ice of quality levels between 80–96% purity. It was observed that the pump speed was directly proportional to the effluent flow rate coming out of the heat exchangers. Generally, it was observed that the quality of product ice was higher at low to medium flow rates of feed. The gear motor speed variation was inversely proportional to effluent flow rate from the heat exchangers. The HybridICE was found to be a viable desalination technology in terms of quality of water produced, energy consumption, and its easiness to be incorporated into existing refrigeration systems.

HybridICE® filter: ice separation in freeze desalination of mine waste waters.

Freeze desalination is an alternative method for the treatment of mine waste waters. HybridICE(®) technology is a freeze desalination process which generates ice slurry in surface scraper heat exchangers that use R404a as the primary refrigerant. Ice separation from the slurry takes place in the HybridICE filter, a cylindrical unit with a centrally mounted filter element. Principally, the filter module achieves separation of the ice through buoyancy force in a continuous process. The HybridICE filter is a new and economical means of separating ice from the slurry and requires no washing of ice with water. The performance of the filter at a flow-rate of 25 L/min was evaluated over time and with varied evaporating temperature of the refrigerant. Behaviours of the ice fraction and residence time were also investigated. The objective was to find ways to improve the performance of the filter. Results showed that filter performance can be improved by controlling the refrigerant evaporating temperature and eliminating overflow.

Deposition of toxic metal particles on rough nanofiltration membranes

Two nanofiltration (NF90 and Nano-Pro-3012) membranes were investigated for their capacity to remove metal ions. This study presents the effect of membrane roughness on the removal of toxic metal ions during dead end membrane filtration. Atomic force microscopy, scanning electron microscopy, WSXM software and ImageJ were used to characterize the roughness of the membranes. Gradual decrease in filtration permeate flux was observed as foulants accumulated at the interface of the membranes; filtration permeate flux varied from 20 L/m2/h to 14 L/m2/h and 11 L/m2/h to 6 L/m2/h for NF90 and Nano-Pro-3012, respectively. NF90 membrane was more prone to fouling than the Nano-Pro-3012 membrane: the percentage flux reduction was higher for NF90 (3.6%) than Nano-Pro-3012 (0.98%). The bearing ratio of the fouled NF90 exhibited a high peak of 7.09 nm than the fouled Nano-Pro-3012 with the peak of 6.8 nm.

Recovery of Calcium Carbonate from Wastewater Treatment Sludge Using a Flotation Technique

The use of flotation technique for the recovery of calcium carbonate (CaCO 3 ) from wastewater treatment sludge was investigated in this study. The parameters that were investigated included dosage of floating agents (sodium oleate and sunlight dish liquid) and the percentage solids of the slurry. The experiments were performed by floating sieved and un-sieved materials and CaCO 3 was determined for both conditions as well as from tailings. Initial CaCO 3 analysis for the bulk material indicated that sieved and un-sieved materials had 63.4% and 32.9% CaCO 3 content by weight respectively. The modification of pH was effected by dosing 1g NaCO 3 to both 1000 g of sieved and un-sieved materials which was sufficient to raise the pH of the slurry to 9.5. A lower average recovery of 2.33% was observed on un-sieved material after using sodium oleate as a collector when compared to sunlight liquid of 31.6%. Therefore, it was concluded that for un-sieved material sunlight dishwashing liquid was a better collector compared to the latter. The results of this study proved that there is great potential of recovering commercial grade limestone from wastewater sludge.

HybridICE® HIF Filter:Principle And Operation

The HybridICE technology operates on the principle that growing ice crystals reject impurities during freezing and is a “zero liquid discharge” process, whereby the water is completely isolated from the dissolved waste species. The technology recovers water from waste waters for re uses for all purposes. The process allows the utilisation of both surplus process heat and cooling energy. The waste heat from the refrigeration cycle is, moreover, utilised for vacuum evaporation to recover a fraction of the water as condensate. The predominant water fraction is recovered by isolating the ice from a concentrated process brine stream. The process takes place in a static concentrator, known as the HybridICE Filter module (HIF) that separates the suspended ice crystals from the concentrated brine slurry to recover ice crystals as pure water. The recovered ice from the freeze crystallisation does not require rinsing with fresh water. Basic factors influence the quality and yield of recovered water. These include but are not limited to: TDS of the waste water stream; first ice point; ice content of the process waste water; mass-flow. The slurry ice feed stream was generated using the HybridICE freeze crystallisation plant. The objective was to establish the comparative behaviour of a low and high sodium chloride feed using 2% and 8% (m/m) NaCl feed brine streams.

Air Quality Monitoring in Metropolitan Harare, Zimbabwe

Recent studies have linked air pollution in cities to chronic health problems like cardiovascular and cardio-respiratory deaths in the population. Pollution of the atmosphere in Harare, the capital of Zimbabwe, is a source of concern. In this study four pollutants (SO2, NO2, Pb, and total suspended particulate matter (TSPM)) were monitored at eight different sites scattered throughout the city for three months (July, August and September). SO2 was determined by bubbling the air into a solution of H2O2, followed by titration. The highest SO2 pollution of 820.0 μg/m3 was in an industrial area and the lowest pollution of 5.0 μg/m3 was in the Central Business District (CBD). SO2 pollution was generally above the World Health Organization (WHO) 24-hour guideline value of 125 μg/m3. NO2 was sampled from the air by passive samplers followed by spectrophotometric determination. The highest NO2 pollution was 46.14 μg/m3 at a site with a busy road nearby and the lowest was 11.09 μg/m3 in a high population residential area. NO2 pollution was generally lower than the WHO guideline value of 40.0 μg/m3 (annual mean). The total suspended particulate matter (TSPM) was determined as “black smoke” using the Soiling Index method. The highest total suspended particulate matter was 154.31 μg/m3 found in a high population density suburb located near some industries and lowest was 9.54 μg/m3 in a low density residential area which is far from pollution sources. Lead was determined by Graphite Furnace Atomic Absorption Spectrometry (GFAAS) for each month for every site and ranged from 0.01 to 0.72 μg/m3. The level of Pb pollution was highest in July and lowest in September at all sampling sites. A positive correlation was found between the levels of Pb and TSPM.