Removal of Polar and Emerging Organic Contaminants by Alternative Adsorbents.

Abstract

The removal of polar and ionizable organic contaminants from drinking water sources is a challenge for water utilities. Prescription and non-prescription drugs are present at detectable levels in many US surface waters, and conventional water treatment processes do not provide an effective barrier for many emerging contaminants. Therefore, effective treatment technologies are required to successfully remove these contaminants from drinking water. In this study the effectiveness of alternative adsorbents for the removal of MTBE was evaluated. Additionally, adsorbent, adsorbate, and water characteristics that affect ionizable organic contaminant adsorption were identified.

In the first phase of this research, the MTBE removal effectiveness of activated carbon, a carbonaceous resin and a silicalite zeolite was compared. Isotherm and short bed adsorber tests were conducted in ultrapure water and river water to obtain parameters describing MTBE adsorption equilibria and kinetics and to quantify the effect of natural organic matter (NOM) on MTBE adsorption. Both the silicalite zeolite and the carbonaceous resin exhibited larger MTBE adsorption uptakes than the tested GAC. Results showed that GAC was the most cost-competitive adsorbent when considering adsorbent usage rate only; however, the useful life of an adsorber containing silicalite zeolite was predicted to be ~5-6 times longer than that of an equally sized adsorber containing GAC. Pilot column results also showed that NOM preloading did not impair the MTBE removal efficiency of the silicalite zeolite. Thus, it may be possible to regenerate spent silicalite with less energy-intensive methods than those required to regenerate GAC.

One activated carbon, one carbonaceous resin, and two high-silica zeolites were studied to evaluate their effectiveness for the removal of an emerging pollutant of concern (EPOC) mixture from lake water. Adsorption isotherm experiments were performed with environmentally relevant concentrations of the 28 targeted EPOCs (~200-900 ng/L). Among the tested adsorbents, activated carbon was the most effective, and activated carbon doses typically used for taste and odor control in drinking water (1-10 mg/L) were sufficient to achieve a 2-log removal for most of the tested EPOCs. The results of this study demonstrate that heterogeneity in pore size and shape along with a large pore volume in the 6-9 Ã… size range are important adsorbent characteristics when an effective barrier against a broad spectrum of EPOCs is desired.

Five carbonaceous adsorbents were evaluated to adsorbed sulfamethoxazole (SMX) and trimethoprim (TMP) at different solution pHs. Results indicated that activated carbon AC1230C more effectively removed both compounds from ultra-pure water (UPW), Tar River water (TRW), and Lake Mead water (LMW). TMP and SMX were more effectively removed at pHs at which both SMX and TMP were primarily in the neutral form. Normalizing the equilibrium liquid-phase concentration of SMX and TMP by the pH-dependent solubility of the compounds was effective. Only at solution pHs where repulsive electrostatic interactions proved to be important the normalization procedure was not able to account for the pH-dependency of the adsorption capacity of the ionizable organic compounds.

Finally, the effect of preloading reduced TMP and SMX adsorption capacities similarly at pH 7.8. SMX adsorption capacities for fresh F400 GAC at pH 3.6 and 5.8 were similar. In contrast for the preloaded GAC the SMX adsorption capacity at pH 5.8 was lower than at pH 3.6. This was related to the decrease in pHPZC and increase in acidic groups of the activated carbon surface with preloaded NOM.