Bromide Removal from Surface Waters by Silver Impregnated Activated Carbon (2015)

Bromide is a precursor of disinfection by-products (DBPs) during potable water treatment, because it causes the formation of brominated DBPs that are more cyto- and geno-toxic than their chlorinated analogues. Due to the potential health risks of DBPs, the United States Environmental Protection Agency (USEPA) has been imposing increasingly stringent regulations for controlling the DBPs. Therefore, removal of bromide ions from source waters becomes critical for controlling the DBPs formation. In my study, it was hypothesized that silver impregnated activated carbon (SIAC) can be a promising adsorbent for bromide removal from water. The main research objectives of this study were to: (i) conduct a systematic investigation for developing a fundamental understanding of SIAC preparation for enhanced removal of bromide from natural waters and (ii) investigate the impact of aqueous background composition (i.e., the presence of natural organic matter and competing anions like chloride, nitrate and sulfate) on the bromide uptake. The study had three sub-objectives: The first objective was to determine the optimum silver impregnation protocols and assess the role of carbon characteristics (e.g., activated carbon characteristics, silver impregnation techniques and pre-oxidation conditions) to produce SIACs to remove Br- from natural waters; and evaluate their effectiveness for Br- removal. The second objective was to evaluate the developed SIACs at typical water treatment conditions. This task involved mainly investigating the competing anions (e.g. Cl-) and NOM and the background matrix effect in natural water samples (e.g. pH, TOC, SUVA, presence of alum during coagulation/flocculation, etc.) The third objective was to evaluate the impact of selected SIAC for DBPs formation control (i.e., THMs control) in natural waters. Several combinations of preparation pathways were employed to prepare SIACs (AC pre-oxidation with 10N and 15.7N nitric acid at 90 ℃ and 160 ℃ followed by silver impregnation with 0.1, 0.5, 1.5M silver nitrate) and to examine the adsorption of bromide from natural waters. Pre-oxidation with 15.7N HNO3 at 90 ℃ and silver impregnation with 0.5M AgNO3 were found to be an appropriate preparation method. In addition, two commercially available SIACs (1.05% and 4.03% silver contents by weight) showed significantly higher bromide uptakes then the virgin activated carbon (AC) in distilled and deionized water (DDW). The enhanced bromide uptake was attributed to the presence of silver species on the carbon surface. The low silver bromide solubility (KspAgBr = 5.2Ã—10-13), and large carbon surface area enabled the SIACs to adsorb bromide from water solution. However, the removal efficiency was affected by the background aqueous conditions (e.g. Cl- concentration, presence of NOM, and other background matrix), silver impregnation process (e.g. silver content, with/without pre-oxidation of virgin AC) and AC characteristics (e.g. surface area, oxygen content). The adsorption experiments showed that SIACs with higher silver contents and larger carbon surface areas exhibited a high degree of bromide removal. The Cl- and natural organic matter (NOM) competition and the water background matrix were important interferences for Br- removal by SIACs. Less Br- removal was observed as the Cl- concentration in the background solution increased; the decreased removal was mainly due to the Cl- competition because the Cl- and Br- competed for silver in the SIAC carbon pores. Br- removal was inhibited in the presence of 2.5 mg/L NOM, which can be attributed to the NOM competition and the pore blockage.