Effects of photodegradation of dissolved organic matter on the binding of benzo(a)pyrene

Abstract

Dissolved organic matter (DOM) in natural waters can bind various organic pollutants, and the affinity of this binding is strongly influenced by the chemical characteristics of the DOM and water pH. This study examined the effects of photochemically induced alteration of the DOM’s chemical properties and water pH on the binding of benzo(a)pyrene (BaP). Time- and pH-series of solar-simulated irradiations were performed on a natural water sample and aqueous DOM solutions prepared from aquatic and soil humic substances. The binding affinity of BaP, expressed as a partition coefficient of a compound to DOM, decreased substantially after the DOM samples were irradiated over environmentally relevant radiation doses and pH ranges. The lowering of the pH due to the photoproduction of acidic products often partly offsets the reduction of the binding affinity caused by direct photoalteration of the DOM’s chemical structure. The decrease of the binding affinity, after correction for the photoinduced pH change, was positively correlated with the decrease in the molecular weight and the aromaticity of the DOM in the course of irradiation. Increasing O₂ abundance accelerated the decrease of the binding affinity as a result of enhanced DOM photodegradation. Visible light played a more important role in reducing the molecular weight and aromaticity of the DOM than in reducing the content of dissolved organic carbon (DOC) via photoremineralization while the reverse was true for UV radiation, indicating that photochemical reduction of the binding affinity may occur in natural waters at depths greater than UV radiation can reach. A decrease of the affinity of DOM for binding BaP will increase the free dissolved fraction of BaP and thus its availability and toxicity to aquatic organisms. The results from this study may have similar implications for organic pollutants other than BaP.

Introduction

Dissolved organic matter (DOM) in natural waters can bind various organic pollutants delivered into aquatic environments (e.g., McCarthy and Jimenez, 1985, Chiou et al., 1986, Kulovaara et al., 1992, Vinken et al., 2005, Tremblay et al., 2005). This binding affects the transport and fate of the pollutants, and hence their bioavailability and toxicity (Kukkonen et al., 1990, Kukkonen and Oikari, 1991). The binding affinity, usually expressed as a partition or binding coefficient, K_oc, is controlled by the physicochemical properties of the DOM (e.g., molecular size, aromaticity, polarity, alkyl moieties) (Chin et al., 1997, Gunasekara and Xing, 2003, Tanaka et al., 2005), the chemical characteristics of the pollutant of interest (e.g., hydrophobicity), and some water chemistry variables (e.g., pH, ionic strength, water hardness) (Schlautman and Morgan, 1993, Penttinen et al., 1998). Therefore, naturally occurring processes that significantly modify the properties of DOM and water chemistry can have a profound impact on the affinity of DOM for binding organic pollutants and thereby on their environmental and ecological ramifications.

Photodegradation is one of the most important processes occurring in natural waters that chemically modify DOM. For example, photodegradation reduces DOM’s average molecular size (Fukushima et al., 2001) and aromaticity (Vähätalo et al., 1999), alters its hydrophobicity (Kulovaara et al., 1996), and lowers pH (Gao and Zepp, 1998, Xie et al., 2004). A reduction in molecular size and aromaticity usually decreases K_oc while changes in pH or hydrophobicity either increase or decrease K_oc, depending on the chemical characteristics of the pollutants (Kukkonen, 1995).

The nature and extent of DOM photoprocessing are affected by various environmental factors, such as O₂ levels, the concentration and speciation of some metal ions (e.g., Fe and Cu) (Gao and Zepp, 1998, Voelker et al., 2000), and the intensity and wavelength of incident light. Raising O₂ levels accelerates the rate of photodegradation and increases the O/C atomic ratio of photoprocessed DOM (Schmitt-Kopplin et al., 1998). DOM photodegradation is also strongly wavelength dependent, with ultraviolet (UV) radiation usually being more efficient than visible light (Gao and Zepp, 1998, Vähätalo et al., 2000). Therefore, these environmental factors can exert significant influences on DOM’s organic binding properties.

To the best of our knowledge, this is the first study to investigate the impact of the photodegradation of DOM on its affinity for binding organic pollutants. We tested one model compound, benzo(a)pyrene (BaP), using a natural DOM sample and aquatic and soil humic substances obtained from different geographic regions. BaP represents an environmentally important class of pollutants, i.e., polycyclic aromatic hydrocarbons. We also examined the effects of O₂ abundance and the wavelength of incident light on the photoalteration of K_oc. The environmental and ecological implications of the photochemically induced changes of K_oc are discussed.