Mark M. Benjamin

MULTIPLE-SITE ADSORPTION OF CD, CU, ZN, AND PB ON AMORPHOUS IRON OXYHYDROXIDE

Abstract Adsorption of Cd, Zn, Cu, and Pb onto amorphous iron oxyhydroxide was measured as a function of pH, metal ion concentration, and adsorbent concentration. For each metal, there is a narrow pH band where fractional adsorption increases from near nil to near 100%. For fixed adsorbent concentration, the pH region of the pH-adsorption edge is independent of total adsorbate concentration when adsorption density is less than 10 −5.0 , 10 −3.7 , and 10 −2.3 moles adsorbate per mole Fe for adsorption of Cd, Cu, and Zn, respectively. At larger adsorption densities for these three metals and over the entire range of adsorption densities studied for Pb, the pH region of the adsorption edge becomes more alkaline as total adsorbate concentration increases. In no case did adsorption density attain a maximum, limiting value. The results suggest that the surface is composed of many groups of binding sites. The strength of binding between a given metal and the surface may vary by an order of magnitude or more from one site to another. At small adsorption densities all types of sites are available in excess, and adsorption can be described by the Langmuir isotherm. However, at higher adsorption densities, availability of the strongest binding sites decreases, leading to a decrease in the apparent adsorption equilibrium constant. This phenomenon occurs under conditions where only a few percent of all surface sites are occupied, and is inconsistent with available single-site models.

Monitoring the properties of natural organic matter through UV spectroscopy: A consistent theory

UV spectra of natural organic matter (NOM) can be represented by three bands, each a Gaussian function of energy. The bands, referred to as the local-excitation (LE), benzenoid (Bz) and electron-transfer (ET) bands, can be ascribed to three types of electronic transitions typical in aromatic compounds. At wavelengths from 200 to 400 nm, an adequate fit to the spectrum can be obtained by consideration of only the Bz and ET bands. The parameters of these two bands are sensitive to the composition of NOM and to alterations in NOM caused by coagulation and chlorination. Removal of NOM fractions with higher-than-average concentrations of carboxyl-, hydroxyl- and ester-substituted aromatic rings or oxidation of those fractions by chlorine decreases the intensity and width of the ET and Bz bands. The ratio of absorbances at the band maxima is an indicator of the average degree of activation of the aromatic rings and permits prediction of the reactivity of the aromatic moiety in chlorination reactions. In addition, measurements of absorbance changes induced by chlorination permit one to monitor certain aspects of chlorination reactions in situ and to evaluate the formation of disinfection by-products.

Sorption and filtration of metals using iron-oxide-coated sand

Abstract Iron oxides are good adsorbents for uncomplexed metals, some metal-ligand complexes, and many metal oxyanions. However, the adsorbent properties of these oxides are not fully exploited in wastewater treatment operations because of difficulties associated with their separation from the aqueous phase. This paper describes experiments in which iron oxides were coated onto the surface of ordinary filter sand, and this composite media was used in a fixed bed configuration for simultaneous filtration of particulate matter and sorption of dissolved metals. The process was successful in removing uncomplexed and ammonia-complexed cationic metals (Cu, Cd, Pb, Ni, Zn), as well as some oxyanionic metals (SeO 3 , AsO 3 ), from simulated and actual waste streams over a wide range of metal concentrations. The adsorbent was stable during backwashing and regeneration operations, releasing most metals quantitatively; the exception was AsO 3 , which was not efficiently recovered by regeneration with either acid or base. The composite media is inexpensive to prepare and could serve as the basis of a useful metal removal and possibly metal recovery process in a variety of settings.

Conceptual model for metal-ligand-surface interactions during adsorption.

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Competitive adsorption of cd, cu, zn, and pb on amorphous iron oxyhydroxide

Abstract Competitive adsorption of Cd, Cu, Pb, and Zn onto amorphous iron oxyhydroxide (Fe 2 O 3 · H 2 O (am)) was studied in systems containing pairs of metal adsorbates. The concentration of the more strongly binding metal was 10–100 times that of the other metal in the system. The conditions were such that if the metals competed for the same group of surface sites, adsorption of the weaker-binding metal should have been significantly reduced when the second metal was added to the system. Nevertheless competitive interactions were minimal, indicating that many of the strong binding sites for one metal are not preferred binding sites for other metals. These results support the hypothesis that the Fe 2 O 3 · H 2 O (am) surface consists of several distinct groups of binding sites. A differential adsorption equilibrium constant is defined and related to the overall average adsorption equilibrium constant. Based on this relationship, the binding constants for a single metal to different surface sites range over at least two orders of magnitude. The differential constant can be used to assess competitive interactions. For instance, when 5 × 10 −5 M Cu and 5 × 10 −7 M Cd compete for sites on Fe 2 O 3 · H 2 O (am) (1 × 10 −3 M Fe T ), less than 2% of the adsorbed Cu binds to “preferred” Cd-binding sites. Failure to account accurately for competitive effects can lead to serious errors when modeling trace metal behavior in natural systems and engineering processes.

Adsorption of natural organic matter (NOM) on iron oxide: Effects on NOM composition and formation of organo-halide compounds during chlorination

A large fraction of the natural organic matter (NOM) in potable water sources can be extracted by adsorption onto iron-oxide-coated sand (IOCS). In this research, two water sources were characterized with respect to their hydrophobic/hydrophilic fractionation, their 13C NMR and UV absorbance spectra and their reactivity with chlorine, before and after the waters were contacted with IOCS. The IOCS preferentially adsorbs acidic fractions of NOM and NOM molecules that are enriched in aromatic and carboxylic carbon. The yield of chloro-organic compounds (quantified as total organo-halide) produced upon chlorination of the NOM or its major fractions correlates with the specific absorbance (As) at 254 nm. IOCS may be useful for removing disinfection by-product (DBP) precursors from potable waters and/or as a medium for NOM accumulation.

Role of organic acidity in sorption of natural organic matter (NOM) to oxide surfaces

Abstract Very strong organic acids (acid groups ionized at pH 3.0) represent a significant portion of the total acidity (4–25%) in samples of organic matter isolated from two lakes, a landfill leachate, and a Krafft mill effluent. On the basis of model compound behavior and established complexation constants, these very strongly acidic groups are predicted to be the key to the formation of strong surface complexes between organic molecules and oxide surfaces. This prediction was confirmed for the sorption of aquatic natural organic matter (NOM) on to preformed iron oxide surfaces. A significant fraction of the carboxylic acidity in polymeric analogs to NOM is due to weak acids (which ionize above pH 8.0). If a portion of the weak acidity in aquatic NOM currently attributed to phenols were actually due to carboxylic acids, significant discrepancies in the understanding of NOM behavior could be reconciled.

The decrease of UV absorbance as an indicator of TOX formation

Abstract The decrease in UV absorbance (ΔUV) caused by chlorination of NOM correlates linearly with the amount of TOX formed, for a remarkably wide range of water quality conditions and reaction times. THM formation also correlates with ΔUV, but the relationship is more sensitive to solution pH and is probably not linear over the entire range of conditions/times relevant for water treatment. It appears that all Cl-DOC reactions that destroy UV absorbance generate TOX, but substantial amounts of THMs are formed only after a significant amount of reaction has occurred.

Correlations between differential absorbance and the formation of individual DBPs.

This study examined correlations between the differential absorbance at 272nm (deltaA272) and the formation of disinfection by-products (DBPs) in chlorinated water from the Tolt River, a water source for Seattle, WA. The DBPs investigated included chloroform (CHCl3), dichlorobromomethane (CHCl2Br), mono-, di- and trichloroacetic acids (MCAA, DCAA, and TCAA, respectively), chloral hydrate (CH), dichloroacetonitrile (DCAN) and 1,1,1-trichloropropanone (TCP). Whereas the kinetics of DBP formation are complex and are non-linear, the same DBP data represented as a function of deltaA272 are quite simple. Absorbance decreases when the water is chlorinated, i.e., deltaA272 is always negative. The DBP vs. -deltaA272 correlations can almost always be quantified by linear equations, at least above some threshold value of -deltaA272, with R2 values > 0.95. The only DBP that did not follow this trend was CH, for which an exponential relationship better described the data. TCP and DCAN were unstable at pH 7 and 8, but at pH 6 linear correlations between their concentrations and -deltaA272 were as strong as those for the more stable DBPs. The threshold -deltaA272 value is approximately the same for many of the DBPs studied, supporting the hypothesis that individual DBPs are released following the formation of a common intermediate, or at least a small group of similar intermediates. The DBP vs. -deltaA272 correlations may have practical value since they provide an alternative approach for monitoring the formation of individual DBP species on-line, but the generality of the relationships needs to be further examined.

Effects of Thermal Treatment on Halogenated Disinfection By-Products in Drinking Water

The influence of heating or boiling on the formation and behavior of disinfection by-products (DBPs) was investigated in DBP-spiked reagent water, municipal tap water, and synthetic water containing chlorinated aquatic humic substances. Thermal cleavage of larger halogenated species leads to both formation of smaller chlorinated molecules (including THMs and HAAs) and dechlorination of organics. In parallel with their formation from larger molecules, THMs can be volatilized, and this latter process dominates the change in their concentration when water is boiled. HAAs are not volatile, but they can be destroyed by chemical reactions at elevated temperatures, with the net effect being loss of trihalogenated HAAs and either formation or loss of less chlorinated HAAs. Although other identifiable DBPs can be generated at slightly elevated temperatures, in most cases their concentrations decline dramatically when the solution is heated.