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Characterization of Carbonaceous Adsorbents by Soil Fulvic and Humic Acid Adsorption

Dep. of Plant and Soil Science, Montana State Univ., Bozeman, MT.

*Corresponding author.

ABSTRACT

Six porous carbonaceous adsorbents, selective for nonpolar organic adsorbates, were evaluated for dissolved soil humic adsorption and testing. Amberlite XAD 8 resin was similarly tested. Soil fulvic (FA) and humic (HA) acids were used as models for naturally occurring dissolved organic C in soil. The adsorbents were tightly packed as spherical capsules identical to previous studies using ion exchange resins for soil adsorption of plant nutrients. Static or passive adsorption from solution was applied. Kinetic studies (0–264 or 0–336 h) indicated a high initial adsorption rate followed by slower adsorption, common for porous adsorbents. Intraparticle diffusion was rate limiting. Adsorption differences between adsorbents are probably caused by molecular-size exclusion effects due to pore size. The capsules of Carboxen-Carbotrap resin mixture were capable of doubling adsorption, requiring no additional time, in response to doubling the FA (43–85 mg C L^–1) or HA (46–90 mg C L^–1) concentration. This effect was seen across the whole adsorption-time range. The constant partitioning isotherm model described adsorption, indicating linear adsorption (r² > 0.96, P < 0.01) with temperatures ranging from 13 to 50 °C, at pH 7.0, and within the concentration ranges of FA (9–126 mg C L^–1) and HA (11–138 mg C L^–1). This suggests a usefulness for these adsorbents at neutral pH and typical temperatures in waters for dissolved humic adsorption. Subsequently, initial solution concentrations of FA and HA were predicted from resin-extractable C, indicating a potential for quantifying humic concentrations in solutions. Under identical conditions, XAD 8 resin did not demonstrate the testing attributes found with these carbonaceous adsorbents.

Contribution of the Montana Agric. Exp. Stn. Journal no. 2761.

Received for publication October 20, 1992.