Vernon L. Snoeyink

Competitive adsorption between atrazine and methylene blue on activated carbon: the importance of pore size distribution

A series of phenolic resin-based microporous activated carbon fibers (ACF) were used to determine how pore size distribution influences the nature of the adsorption competition mechanism between the micropollutant, atrazine, and a compound similar in size, methylene blue (MB). Experiments consisted of simultaneous adsorption, dye preloading, and atrazine preloading. Direct competition for adsorption sites is the primary mode when the competing adsorbate can access the same pore size region as the target micropollutant. When only a narrow distribution of primary micropores (pore width <8 A) is present, simultaneous adsorption and dye preloading greatly impacted atrazine adsorption. Increasing the micropore volume and shifting the pore size distribution into the secondary micropore region (8 A<pore width <20 A) reduced the degree of competition. The relative impact of preloading with MB on atrazine adsorption decreased with increasing pore volume and pore size. When atrazine was preloaded, the low level of atrazine desorption from the smallest pore size adsorbent, which contained mainly primary micropores, provided evidence for strong adsorption/slow desorption in these pores. This is consistent with the enhanced adsorption resulting from overlapping pore wall potentials, contributing to non-ideal competitive adsorption. The displacement of pre-adsorbed atrazine by MB in the other four ACFs is primarily associated with direct competition for sites in the secondary micropore region.

Competitive adsorption in natural water : Role of activated carbon pore size

The impact of pore size on the competition mechanism between natural organic matter (NOM) in Illinois groundwater and the micropollutant atrazine was assessed using activated carbon fibers (ACFs). Two microporous ACFs with narrow and broad pore size distributions, designated ACF-10 and ACF-25, respectively, were used. The average pore sizes of ACF-10 and ACF-25 were 6 and 13.4 A. Single solute adsorption, simultaneous adsorption and preloading experiments were performed. On ACF-10 it was found that the adsorption of atrazine was reduced significantly in the presence of NOM, even though the NOM loading was very small as a result of pore exclusion. The uptake of atrazine by ACF-10 in the presence of NOM (simultaneous adsorption) was comparable to the NOM-preloaded capacity. In addition, preloaded atrazine was not displaced by subsequently adsorbed NOM. The results support a pore blockage mechanism by which NOM molecules block access to, but do not penetrate into the primary micropores. Atrazine capacity on ACF-25 which has primary micropores as well as a large volume of secondary micropores, was reduced in the presence of NOM; however, the reduction in capacity was much less than that observed with ACF-10. Preloading with NOM showed further capacity reduction compared with simultaneous adsorption. These results combined with the result that preloaded atrazine exposed to NOM showed displacement of atrazine support a direct site competition mechanism in the secondary micropore region. Attempts to regenerate NOM preloaded ACF-10 and ACF-25 using a strong alkali solution failed to recover atrazine capacity, suggesting that NOM was strongly adsorbed at the fiber surface as well as within micropores.

Physico-chemical characteristics of corrosion scales in old iron pipes

Corrosion scales play an important role in modifying water quality in drinking water distribution systems. The corrosion scales from old iron/steel pipes were analyzed for their structure and composition. Scales were studied both before and after drying. and goethite, magnetite and lepidocrocite were identified as the primary constituents of the dried samples. High concentrations of readily soluble ferrous phases were detected in wet-scale samples. The corrosion scales had a shell-like, enveloping layer, covering porous deposits of iron oxide phases. Our studies were able to identify important differences between corrosion scales found in two different water distribution systems. Further studies are needed to establish the role of corrosion scales in the mechanism of iron release from corroded pipes.

Characterization and activated carbon adsorption of several humic substances

Abstract A commercially supplied humic acid, extracts of a Michigan Finch soil, and extracts of leaves from a hardwood forest were characterized by molecular size fractionation, functional group analysis, and haloform formation potential. The haloform formation potential was dependent on the source of the material; differences could not be correlated with the amounts of oxygen-containing functional groups in each substance. A bituminous-base activated carbon adsorbed each humic substance with the adsorption capacity at seven days being source-dependent. The extent of adsorption of humic material from various sources was inversely related to the number of carboxyl groups per unit weight of the substance. The lower molecular weight species of a given humic or fulvic acid was more adsorbable, presumably because more surface area was accessible to these species. Soil fulvic acid adsorption increased with decreasing solution pH and increasing phosphate buffer concentration. Due to the variability of haloform formation potential and adsorbability of humic substances from different sources, treatment performance to remove these materials should be evaluated using the specific water to be treated.

A kinetic and equilibrium study of competitive adsorption between atrazine and Congo red dye on activated carbon: the importance of pore size distribution

A series of phenolic resin-based microporous activated carbon fibers (ACF) with different micropore size distributions were used to assess the role of pore size distribution (PSD) in the mechanism of competitive adsorption between the organic micropollutant, atrazine, and a compound larger in size, Congo red dye (CR). Batch kinetic and equilibrium experiments with the CR/atrazine system consisted of single-solute, simultaneous adsorption, CR preloading followed by atrazine contact, and atrazine preloading followed by CR contact. Based on the previous pore characterization studies and the PSD, two types of pore structures were proposed: telescopic pores and branched pores. With the telescopic pore structure, evidence is presented to support a transition from surface pore blockage to pore constriction (without loss of atrazine capacity) to direct competition for adsorption sites, with increasing average micropore size. With the branched pore structure (micropores branching off from mesopores), direct competition for adsorption sites in a fraction of the large micropores and pore constriction and pore blockage of smaller micropores were found to be important. The kinetics of adsorption was found to be important in determining the impact of simultaneous adsorption, while CR surface coverage and preloading time were the key factors controlling the impact of preloading on atrazine adsorption.

Elucidating competitive adsorption mechanisms of atrazine and NOM using model compounds

Based on the relative adsorbability of natural organic matter (NOM) fractions with different molecular weights (MWs), two model compounds, poly(styrene sulfonate) (PSS) (nominal MW=1800 Dalton) and p-dichlorobenzene (DCB), were chosen to study the competitive effect of large and small NOM molecules on atrazine adsorption by two powdered activated carbons (PACs) with different pore size distributions. Both isotherm and kinetic tests of atrazine adsorption were conducted using fresh PAC and PAC preloaded with the model compounds. The model compounds were found to affect atrazine adsorption through two different mechanisms due to their size difference: direct competition for sites by p-DCB and pore constriction/blockage by PSS-1.8k. p-DCB was found to significantly reduce atrazine adsorption capacity but to have no effect on atrazine adsorption kinetics. In contrast, the effect of PSS-1.8k on atrazine adsorption capacity was very small. Furthermore, during simultaneous adsorption, PSS-1.8k had no effect on atrazine surface diffusion. However, preloading PAC with PSS-1.8k lowered the atrazine surface diffusion coefficient, D(s), by more than three orders of magnitude; D(s) decreased with increasing solid phase PSS-1.8k concentration. The pore size distribution of the PAC was found to play an important role in competitive adsorption. A high mesopore surface area could alleviate pore blockage significantly.

The reduction of bromate by granular activated carbon

Abstract The removal of bromate, an inorganic disinfection by-product, by granular activated carbon (GAC) was investigated in this study. Bromate ion removal from water was observed in the presence of virgin and acid-washed outgassed (AWOG) GAC. In a GAC filter with distilled-deionized water, bromate breakthrough occurred slowly whereas bromate breakthrough occurred very quickly in natural water due to the presence of natural organic matter (NOM) and other anions. NOM adsorption decreased bromate reduction, presumably by blocking bromate reduction sites. The use of a biologically active carbon (BAC) filter with ozonated water, as a pretreatment step to remove NOM, only slightly improved bromate reduction in the subsequent fresh GAC filter. Kinetic studies showed that the presence of chloride, sulfate, bromide, and nitrate causes a decrease in the kinetics of bromate reduction by GAC. These anions may occupy ion exchange sites on the carbon, reducing the rate at which bromate can access the reduction sites. However, when the anions were released from the carbon, the bromate reduction rate increased.

WATER QUALITY FACTORS AFFECTING BROMATE REDUCTION IN BIOLOGICALLY ACTIVE CARBON FILTERS

Biological removal of the ozonation by-product, bromate, was demonstrated in biologically active carbon (BAC) filters. For example, with a 20-min EBCT, pH 7.5, and influent dissolved oxygen (DO) and nitrate concentrations 2.1 and 5.1 mg/l, respectively, 40% bromate removal was obtained with a 20 microg/l influent bromate concentration. In this study, DO, nitrate and sulfate concentrations, pH, and type of source water were evaluated for their effect on bromate removal in a BAC filter. Bromate removal decreased as the influent concentrations of DO and nitrate increased, but bromate removal was observed in the presence of measurable effluent concentrations of DO and nitrate. In contrast, bromate removal was not sensitive to the influent sulfate concentration, with only a slight reduction in bromate removal as the influent sulfate concentration was increased from 11.1 to 102.7 mg/l. Bromate reduction was better at lower pH values (6.8 and 7.2) than at higher pH values (7.5 and 8.2), suggesting that it may be possible to reduce bromate formation during ozonation and increase biological bromate reduction through pH control. Biological bromate removal in Lake Michigan water was very poor as compared to that in tapwater from a groundwater source. Bromate removal improved when sufficient organic electron donor was added to remove the nitrate and DO present in the Lake Michigan water, indicating that the poor biodegradability of the natural organic matter may have been limiting bromate removal in that water. Biological bromate removal was demonstrated to be a sustainable process under a variety of water quality conditions, and bromate removal can be improved by controlling key water quality parameters.

The effect of preloading on rapid small-scale column test predictions of atrazine removal by GAC adsorbers

Abstract Rapid small-scale column tests (RSSCTs) were evaluated for their ability to predict atrazine removal in pilot-scale granular activated carbon (GAC) adsorbers. The performance of both virgin and preloaded GACs was tested. Atrazine removal by virgin GAC was studied in post-filter adsorbers at Toulouse, France, and Choisy-le-Roi, France, using empty-bed contact times (EBCTs) of 10.3 and 14 min, respectively. For virgin GAC, RSSCTs successfully simulated atrazine removal over large-scale operating times of about 3.5–7 months. However, RSSCTs significantly overestimated atrazine removal at longer operating times. Atrazine removal by preloaded GAC was studied in pilot-scale post-filter adsorbers at Choisy-le-Roi, France, after preloading times of 5 months and 20 months. EBCTs were approximately 8.5 min. To describe the performance of pilot-scale adsorbers containing preloaded GAC, RSSCTs were initiated with virgin activated carbon, and preloading was simulated prior to the spiking of atrazine. For a pilot-scale adsorber containing GAC that had been preloaded for 5 months, the RSSCT data effectively described atrazine removal. However, the RSSCT was not successful in predicting atrazine removal by GAC after a preloading time of 20 months. Discrepancies between RSSCT and pilot data for long service times or after extended preloading periods may have been due to enhanced adsorption of background organic matter in the presence of oxygen. Overall, RSSCTs were judged to be most useful for predicting the initial performance of adsorbers containing virgin GAC.

Reaction mechanism of calcium-catalyzed thermal regeneration of spent granular activated carbon

Thermal regenerations were conducted on a spent granular activated carbon (GAC) that had served in a water treatment plant for about four years and contained 1.8% calcium. Oxidation was conducted in steam, CO2, or steam plus CO2, at 650–950°C. As described earlier, when steam and CO2 were employed together, the regenerated products pore structure maintained greater micropore volume at high temperatures ( > 800°C) than at low temperatures (<750°C). The work herein has linked this distinction to differences in reaction rate limitations: at high temperatures, with both oxidants employed together, the water-gas shift reaction (H2O + CO = H2 + CO2) limited overall rate, whereas at low temperatures, the C(O) gasification step limited overall rate. The CO2 created via the water gas shift reaction served as the primary oxidant. Although steam served only as a secondary oxidant, its presence also facilitated more extensive oxidant transfer to the carbon surface.