Carbon literature is rich in methods on carbon synthesis and surface modification to prepare
materials with desired physicochemical properties for a particular type of application. Our
understanding about dissolved organic matter (DOM) and its adsorption by granular activated carbon (GAC) have greatly improved within the past
two decades. Therefore, the main objective of this research was to develop tailored activated
carbons for enhanced and selective removal of DOM from natural waters. Based on the large
size of the DOM, it is hypothesized that the proper type of adsorbents for this application
should have a sufficient amount of mesoporosity (i.e., a larger fraction of pore volume and
surface area should be in the pores >2nm). At the same time, DOM is a heterogenous mixture
of polar molecules with negatively charged and acidic functionalities in natural waters. Due to
their hydrophilic nature, solvent-motivated adsorption (i.e., hydrophobicity effect) is not
expected to play a major role in the removal of DOM molecules by GAC from water.
Therefore, it is further hypothesized that for enhanced and effective DOM removal, the carbon
surface should have basic properties, carry an overall positive charge, carry specific
functionalities, or an optimum combination of all these characteristics.
It is possible to control carbon porosity by either selecting starting materials with different
porosities or enlarging micropores of a sorbent using different treatment schemes. Both
approaches were used in this study. Three GACs (microporous F400, mesoporous WVBMESO
and WVB-MACRO) were selected as sorbents. Obtained results of virgin and
modified F400 carbons are presented and discussed in this paper. A similar study of
mesoporous carbons is in progress.
Each carbon surface was modified using three main approaches: heat treatment under He or
H2 atmosphere; NH3 treatment at low and high temperatures with or without pre-oxidation
with HNO3; and, iron impregnation. Heat and ammonia treatments enhance the basicity of
carbon surface. In addition, ammonia treatment at different temperatures introduces various
nitrogen-containing functionalities to the carbon surface, which may interact with DOM
components. Oxidation followed by ammonia treatment enlarges carbon pores while
increasing the value of surface basicity and the number of nitrogen-containing functionalities.
Iron cations, or even zero-valent iron, dispersed on the carbon surface can significantly
enhance affinity of DOM components toward the carbon surface. Recent results obtained
during the development of enhanced coagulation guidelines indicated that it is possible to
remove more DOC using iron coagulants, compared with aluminum coagulants (Randtke, et
al., 1999; Edwards, 1997). Data indicated also that some fractions of non-removable DOM by
alum could be removed by iron chloride. Therefore, it is expected that DOM uptake by GACs
be increased by promoting: electrostatic attractions between cationic iron species and
anionic DOM molecules; and/or favorable complex formation between the iron and DOM
species.
Since various treatment routes have been used to prepare different sorbents, the modified
carbons were named with abbreviation codes describing the treatment procedures employed:
He: heat treatment at 900C for 2hr under helium flow; H: heat treatment at 900C for 2hr
under hydrogen flow; 16NO: oxidation by using boiling concentrated nitric acid for 1hr;
8N2H: treatment with ammonia at 800C for 2hr; 4N1H: treatment with ammonia at
400C for 1hr; and, Fe3E and FeS: iron impregnation by ion exchange and incipient
wetness methods, respectively. For example F400-He-16NO is the F400 carbon that was heat
treated under helium and then oxidized with concentrated nitric acid. Virgin and modified
carbons were characterized by surface area and pore size distribution analysis, pHPZC,
elemental analysis, water vapor adsoption, acid/base uptake, and iron analysis of digested
samples.
DOM samples we
| Edition : | Vol. - No. |
| File Size : | 1
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| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 8 |
| Published : | 06/15/2003 |
