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Target separation of arsenic from contaminated raw water

Arsenic is known to be one of the most toxic and carcinogenic elements known to have affected millions of people worldwide. Arsenic in surface and groundwater originates from both natural and anthropogenic sources. Prolonged exposure to arsenic can lead to skin disease, cancer, diabetes and cardiovascular disease. This thesis investigates target separation of arsenic from contaminated water using the adsorption and coagulation/flocculation/dissolved air flotation (C/F/DAF) processes. DMI-65, a silica based catalytic media was used as an adsorbent to investigate As (III) and As (V) removal from a contaminated drinking water. Batch adsorption studies were conducted at different pH (5, 6, 7 and 8.5) to determine adsorption kinetics. Equilibrium was achieved after 6 hours of contact time and experimental data were best fitted to the pseudo second-order kinetic model for As (III) and As (V). The adsorption data were best fitted to Langmuir isotherm models and the maximum adsorption capacity of DMI-65 for As (III) and As (V) were estimated to be 0.318 mg/g and 0.237 mg/g respectively. Thermodynamics studies revealed that adsorption capacity and arsenic removal percentage using DMI-65 increased with increase in temperature. The adsorption process can be attributed to physisorption as a result of Van der Waals interaction at the surface of the adsorbent. Regeneration and reusability of a media is an important economic factor. DMI-65 was able to demonstrate its ability to regenerate after several cycles using sodium hydroxide (NaOH) and performed well over a wide pH range. The maximum removal efficiency for As (III) was 96.55 % at pH 5 and 90.40 % for As (V) at pH 8.5. A continuous adsorption study was conducted in a fixed-bed column to remove arsenic from the Waikato River using DMI-65. The dynamic adsorption capacity and breakthrough time was well predicted by Thomas and Yoon-Nelson models. The maximum adsorption capacity was found to be 11.96 mg/g using DMI-65 as adsorbent for initial concentration, flowrate and pH of ~ 13.00 – 20.00 µg/L, 20 mL/min and 5, respectively. Results obtained showed that DMI-65 is effective in removing arsenic from contaminated drinking water. It is also effective in reducing turbidity but fouling of the adsorption column is a major drawback and leads to increased pressure drop in the fixed-bed column. Competing ions in the form of organic anions such as nitrate (NO₃⁻), silicate (SiO₄⁴⁻), sulfate (SO₄²⁻), bicarbonate (CO₃²⁻) and phosphate (H₂PO₄⁻) are known to interfere with arsenic removal in natural waters. This interference is also influenced by the pH of the solution and anionic concentration. The study was conducted at pH levels (5 – 9) and anionic concentration strength (1, 5 and 10 mM). The most significant interference with the removal of As (V) by polyaluminium chloride (PAC) occurred in the presence of phosphate at pH 6 with a removal rate of 5.52 % at 10 mM phosphate concentration. Overall, the major impact of the competing anions on As (V) removal followed the following order of H₂PO₄⁻ > CO₃²⁻ > NO₃⁻ > SO₄²⁻. Target separation of arsenic from other contaminants was investigated in a two-stage C/F/DAF process. The results of this study indicated that increase in flotation time (10 – 30 minutes) and saturation pressure (2 – 4 bar) did not result in any significant increase in arsenic removal efficiency. Turbidity removal efficiency was poor at pH 5 and 6 despite recording higher arsenic removal efficiency. The first stage DAF process showed that 88.10 % of arsenic and 3.44 % turbidity removal efficiency was achieved using 9.4 mg/L of PAC at pH 6. Increasing the pH of the remaining solution to 8 in the second-stage DAF process resulted in 83.55 % arsenic removal and 64.08 % turbidity removal after adding 0.47 mg/L PAC. These findings showed that the amount of arsenic leaving the water treatment plant (WTP) to the wastewater treatment plant (WWTP) can be reduced from the current 0.942 kg/day to 0.16 kg/day. This represents an overall reduction of 83.01 % in the amount of arsenic that would have end up in the biosolids thereby making it unfit for agricultural purposes. This result showed that arsenic can successfully be separated from other contaminants in a two-stage C/F/DAF process by change in pH from 6 to 8 and under optimised operating conditions.
Type of thesis
Aremu, J. O. (2020). Target separation of arsenic from contaminated raw water (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13827
The University of Waikato
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