Effects of Activated Carbon Surface Chemistry and Pore Structure on the Absorption of Methyl Tertiary-Butyl Ether and Trichloroethene from Natural Water

Abstract

Activated carbon adsorption is the best available treatment technology for thecontrol of many objectionable trace organic compounds. Activated carbons are frequentlycharacterized by the iodine number and BET surface area, but these parameters do notcorrelate well with trace organic compound removal from natural water. Therefore, theobjective of this research was to develop activated carbon selection criteria that assure theeffective removal of trace organic contaminants from natural water and to base theselection criteria on the adsorbent's pore structure and surface chemistry. Tosystematically evaluate pore structure and surface chemistry effects, a matrix of activatedcarbon fibers (ACFs) with three activation levels and four surface chemistry levels wasstudied. To evaluate whether adsorption trends established for ACFs were also valid forgranular activated carbon (GAC), ACF results were compared with those obtained forthree commercially available GACs. Adsorption capacities were determined for naturalorganic matter (NOM), for relatively hydrophilic methyl tertiary-butyl ether (MTBE) andrelatively hydrophobic trichloroethene (TCE) in organic-free water, and for MTBE andTCE in the presence of NOM. NOM isotherms showed that DOC adsorption occurredprimarily in pores with diameters in the 11 to 500 Å range and that electrostaticinteractions between NOM and the carbon surface played a role in NOM adsorption.According to both single-solute isotherms and micropollutant isotherms in the presence of NOM, hydrophobic adsorbents more effectively removed TCE and MTBE thanhydrophilic adsorbents. Effective adsorbents for drinking water treatment shouldtherefore contain little oxygen and nitrogen whose presence increases the polarity of theadsorbent surface. Based on the elemental composition of the low-ash carbons evaluatedin this study, activated carbons should have oxygen and nitrogen contents that sum to nomore than 2 to 3 mmol/g to assure sufficient hydrophobicity. In addition, both single-soluteisotherms and isotherms in the presence of NOM indicated that adsorbents shouldexhibit a large pore volume in micropores with widths that are about 1.5 times larger thanthe kinetic diameter of the target adsorbate. Furthermore, based on the micropollutantisotherms in the presence of NOM, an effective adsorbent should possess a microporesize distribution that extends to widths that are approximately twice the kinetic diameterof the target adsorbate to prevent pore blockage or restriction as a result of NOMadsorption.