PERCHLORATE SORPTION BY ACTIVATED CARBON: ROLE OF SURFACE FUNCTIONAL GROUPS

Perchlorate is used as a solid oxidant in rocket propellants and in fireworks. It has become a major groundwater contaminant and has attracted serious attention because of its harmful effects on human health: it inhibits iodide uptake by the thyroid gland. Adsorption by activated carbon is one of the potential remediation processes to remove perchlorate from groundwater. In this work, four commercially available activated carbons with similar surface area and pore size distribution are tested for perchlorate sorption capability and the sorption mechanism elucidated. Based on the data on surface functional groups obtained from acid-base titrations, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron emission spectroscopy (X-PEEM) and surface hydrophobicity from water contact angles, the materials can be divided into two types: type I materials with predominantly carboxylic surface functional groups and type II materials dominated by the phenolic surface functional groups. The plots of distribution coefficient K_d vs pH fall into three categories, with two samples (type I) showing high sorption capabilities with the distribution coefficient K_d varying from 6.8 x 10³ to 1.8 x 10⁴ ml/g as the solution pH changes from 8.5 to 3.8, and one sample (type II) showing slightly lower K_d values (4.8 x 10³ and 1.04 x 10⁴ ml/g at solution pH values of 8.5 and 3.8 respectively) and the fourth sample (type II) showing very poor sorption capability over a similar pH range. The slopes of the curves for all four samples are very similar and close to zero indicating a weak pH dependence of sorption, leading to the conclusion that the sorption mechanism is mediated by chemical interactions (inner sphere complexation) and the different distribution (carboxylic vs. phenolic) of surface functional groups rather than by pure electrical or ionic interactions. In conclusion, perchlorate removal by activated carbon can be greatly improved by tailoring the activation and treatment processes and modifying the distribution of surface functional groups.