The effect of co-occurring inorganic solutes on the removal of arsenic (V) from water using cationic surfactant micelles and an ultrafiltration membrane Original Research Article

The effect of co-occurring inorganic solutes existence in feed water on the removal of arsenic (V) and permeate flux was investigated for the treatment process using a cationic surfactant cetylpyridinium chloride (CPC) and a 5 kilo-Dalton (kDa) polyethersulfone (PES) membrane. Simulated water and well water (Washoe County STMGID #9 operational well, Nevada) were used for this study. The concentrations of arsenic ([As]F = 0−105 μg/L) and inorganic solutes ([HCO−3] = 0−4.1 mg/L, [HPO2−4] = 0−0.3 mg/L, [H4SiO4] = 0−90 mg/L, and [SO2−4] = 0−400 mg/L) in simulated feed water were varied. The pH level of simulated feed water was adjusted to 8 since the well water pH was around 8. The arsenic concentration in the well water was 73 μg/L. The concentrations of carbonate, phosphate, silicate, and sulfate in the well water were 1 mg/L, 0.17 mg/L, 69 mg/L, and 8.2 mg/L, respectively. PES membrane without surfactant micelles was found to be ineffective for arsenic removal. The highest arsenic removal in the presence of inorganic solutes is 25%, corresponding to a permeate water arsenic concentration of 30 μg/L which is well above the new maximum contaminant level (MCL) of 10 μg/L. With the addition of 10 mM CPC to the feed water, the arsenic removal efficiency is significantly increased ranging between 78.1% and 100%. The arsenic removal efficiency was found to be dependent on the feed water arsenic and co-occurring inorganic solute concentrations. Except for one UF experiment performed with the simulated feed water containing 105 μg/L arsenic and 400 mg/L SO2−4 (23 μg/L arsenic in permeate water), all the other UF experiments produced permeate water with arsenic below the new MCL. The presence of HCO−3, HPO2−4, and H4SiO4 species in feed water does not affect the arsenic removal efficiency (100%) except in one case (99% removal efficiency for the feed water containing 105 μg/L arsenic and 90 mg/L H4SiO4). The experiments performed with well water result in almost 100% arsenic removal (arsenic level in permeate water is below the detection limit of ICP-MS, 1 μg/L). Furthermore, absolute permeate flux was also found to be dependent on the feed water arsenic and co-occuring inorganic solute concentrations. An increase in feed water arsenic concentration results in a decrease in the permeate flux. For the experiments performed with simulated water containing CPC and arsenic, the permeate water flux is 56.5 ± 2.5% of ultra clean water flux. The permeate flux reduces further as the co-occuring inorganic solute concentration or the total dissolved solid in feed water increases. It is 43.5 ± 3.5%, 45.5 ± 7.5%, 43.5 ± 6.5%, and 37.5 ± 5.5% of the ultra clean water flux for the simulated water containing HCO−3, HPO2−4, H4SiO4 and SO2−4 species, respectively. With the well water, permeate flux was found to be 33% of the ultra clean water flux. Only 1.52 ± 0.33% of the surfactant molecules used were detected in permeate water. Surfactants in permeate water can be further separated and reused in the feed stream.