Environ. Sci. Technol. 2007, 41, 6448-6453
The Effect of Water Temperature on the Adsorption Equilibrium of Dissolved Organic Matter and Atrazine on Granular Activated Carbon BERND SCHREIBER,† VIKTOR SCHMALZ, THOMAS BRINKMANN,‡ AND ECKHARD WORCH* Institute of Water Chemistry, Dresden University of Technology, 01062 Dresden, Germany
The influence of water temperature on the adsorption of natural dissolved organic matter (DOM) on activated carbon has not been investigated intensively yet. In this study, batch experiments with granular activated carbon (GAC) have been carried out at three temperatures (5 °C, 20 °C, 35 °C) using a humic acid model water and different types of surface water (lake, river, canal). Furthermore, the adsorption of an anthropogenic contaminant, atrazine, was quantified in the absence and presence of DOM. The results indicate a significant influence of water temperature on the adsorption equilibrium of DOM and atrazine. Contrary to expectations, DOM and atrazine adsorption in surface water tends to be increased with increasing water temperature, whereas the extent of this effect is dependent on the type and concentration of DOM. Furthermore, the temperature effect on atrazine adsorption is controlled by competition of DOM and atrazine on adsorption sites. Some assumptions are proposed and discussed for explaining the temperature effects observed in the batch studies.
Introduction The adsorption of natural dissolved organic matter (DOM) and organic micropollutants on activated carbon during water treatment is influenced by a number of parameters such as concentration and type of DOM and micropollutant, pH, ionic strength, and properties of activated carbon (1-8). Furthermore, adsorption of organic micropollutants is often reduced by the presence of natural organic matter (NOM) due to competitive adsorption (9-11). There is still little information on the influence of water temperature on adsorption. From a thermodynamic point of view, adsorption based on physical interactions (physisorption) should decrease with increasing temperature, but experimental findings are not so clear. Some investigations have been focused on adsorption of single solutes, e.g., phenols (12-15) and others on adsorption of NOM (3, 4, 16). The results obtained from these adsorption studies are fairly * Corresponding author tel.: +49-351-463-32759; fax: +49-351463-37271; e-mail:
[email protected]. † Present address: ERGO Umweltinstitut GmbH, Lauensteiner Strasse 42, 01277 Dresden, Germany. ‡ Present address: Federal Environmental Agency, FG III 3.4, Schichauweg 58, 12307 Berlin, Germany. 6448
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 18, 2007
diverse and are apparently dependent on the compounds and system parameters chosen for the respective study. In some cases, an augmentation in temperature has been identified to increase adsorption capacity for phenol and o-cresol in oxic systems (13), for humic substances (4), and for DOM (17), while in others an increase in temperature led to a decrease in adsorption capacity for phenols (14), for phenol and o-cresol in anoxic systems (13), or had no significant influence for NOM (16). Additionally, a maximum of adsorption of phenolic compounds was observed by Ravi et al. (15) and Radeke and Hartmann (12) at temperatures of 25 °C and 40 to 50 °C, respectively. Pikaar et al. (18) evaluated the adsorption of 12 compounds on different activated carbons and proposed a two-domain model (highenergy and low-energy sites). Using a dual Langmuir model for describing the isotherms they found that the maximum adsorption capacity of the high-energy sites decreases with increasing temperature and the maximum capacity of the low-energy sites increases with increasing temperature. This study has been implemented to quantify water temperature effects on adsorption of DOM and atrazine as a model micropollutant on granular activated carbon (GAC) using batch experiments. Temperature effects were investigated with special regard to competitive adsorption between DOM and the micropollutant.
Materials and Methods Water Samples. Three surface waters of different origin (lake, canal, and river water), sampled near Dresden and Berlin, Germany, were used. Additionally, adsorption experiments were undertaken with ultrapure water containing Aldrich humic acid sodium salt (AHA) buffered at pH 8.3 by adding 1 mmol/L of NaHCO3. AHA was chosen for comparison because it represents a completely different type of DOM than the original surface water DOM. Immediately after sampling, the surface waters were filtered through glass fiber filters to remove biomass and coarse grains and were then stored for a minimum of three weeks at room temperature to achieve a constant DOC concentration after biodegradation. Activated Carbon. Granular activated carbon (GAC) F 300 obtained from Chemviron Carbon was used throughout the experiments. Prior to the use of the GAC, it was sieved to get a granular fraction of 0.3 to 0.4 mm. The BET surface area of the sieved GAC was found to be 1036 m2/g. Preparation and further properties of the GAC are described elsewhere (17). Batch Experiments. DOC and atrazine isotherms were determined by the bottle-point method (i.e., one initial concentration, different adsorbent masses) according to the situation in practical batch adsorption systems. The water was filtered through cellulose nitrate membranes (0.45 µm) and spiked with NaN3 (c ) 200 mg/L) to suppress microbial activity. Atrazine (Riedel-de-Hae¨n, analytical grade) was added to the water as a model contaminant to an initial concentration of 1 or 2 mg/L, dependent on the equilibrium concentrations to be expected. Amounts of 3 up to 600 mg of GAC were put into steel bags to protect it from destruction during the stirring process. Before starting the experiment, the water was stored at the desired temperature for at least 48 h. For obtaining adsorption equilibrium, the water was stirred for a total period of 21 days at temperatures of 5 °C, 20 °C, and 35 °C. From preliminary kinetic tests, this runtime was found to be sufficient. Prior to the analysis the water was again filtered by a 0.45 µm membrane filter to remove GAC residues. 10.1021/es070704+ CCC: $37.00
2007 American Chemical Society Published on Web 08/15/2007
TABLE 1. Characteristics of the Water Samples name water type/location pH conductivity in µS/cm (ϑ ) 20 °C) DOC in mg/L SAC254 in 1/m SUVA254 in L/(mg‚m) ultrafiltration: >10000 Da 10000-1000 Da
