Enzymatic activities in constructed wetlands and di-n-butyl phthalate (DBP) biodegradation

Abstract

The bioaccumulation of phthalate acid esters (PAEs) from industrial products and their mutagenic action has been suggested to be a potential threat to human health. The effects of the most frequently identified PAE, Di-n-butyl phthalate (DBP), and its biodegradation, were examined by comparison of two small scale plots (SSP) of integrated vertical-flow constructed wetlands. The influent DBP concentration was 9.84 mg l⁻¹ in the treatment plot and the control plot received no DBP. Soil enzymatic activities of dehydrogenase, catalase, protease, phosphatase, urease, cellulase, β-glucosidase, were measured in the two SSP after DBP application for 1 month and 2 months, and 1 month after the final application. Both treatment and control had significantly higher enzyme activity in the surface soil than in the subsurface soil (P<0.001) and greater enzyme activity in the down-flow chamber than in the up-flow chamber (P<0.05). In the constructed wetlands, DBP enhanced the activities of dehydrogenase, catalase, protease, phosphatase and inhibited the activities of urease, cellulase and β-glucosidase. However, urease, cellulase, β-glucosidase activities were restored 1 month following the final DBP addition. Degradation of DBP was greater in the surface soil and was reduced in sterile soil, indicating that this process may be mediated by aerobic microorgansims. DBP degradation fitted a first-order model, and the kinetic equation showed that the rate constant was 0.50 and 0.17 d⁻¹, the half-life was 1.39 and 4.02 d, and the r² was 0.99 and 0.98, in surface and subsurface soil, respectively. These results indicate that constructed wetlands are able to biodegrade organic PAEs such as DBP.

Introduction

Phthalate acid esters (PAEs) are important additives to many industrial products, and occupy 80–85% of the global plasticizer market (Ejlertsson et al., 1997). Studies have shown that PAE contamination in waters, sediments, soils and biota (Staple et al., 1997) may be toxic to mammals and aquatic organisms (Adams et al., 1995, Foster et al., 2000, Patyna and Cooper, 2000). In addition, the high bioaccumulation rate and mutagenic action poses a potential threat to human health.

Di-n-butyl phthalate (DBP) is the most frequently identified PAEs in diverse environmental samples, and it has been listed as a priority pollutant by the US Environmental Protection Agency. DBP is relatively stable in the natural environment. The half-life by photodegradation in natural waters ranges from several months to years (Bajt et al., 2001). Studies of DBP biodegradation in seawater, soil, activated sludge and an expanded granular sludge bed revealed a higher degradation rate (Walker et al., 1984, Narayanan et al., 1993). Unfortunately, these processes have strict conditions and low treatment capacities.

Constructed wetlands provide an efficient ecological system with low maintenance requirements and construction costs. They are able to remove suspended solids, N, P, heavy metals, and organic pollutants, such as PAHs, TNT and benzoic acid (Zachritz et al., 1996). These wetlands are also effective in processing coliforms from municipal sewage, storm water, and agricultural runoff (Lakatos et al., 1997, Cheng et al., 2002). Zhao et al. (2002) found that in integrated vertical-flow constructed wetlands, the average removal rate of DBP was over 99% and DBP concentration in effluents was less than 10 μg l⁻¹. However, studies of the responses of constructed wetlands soils to these pollutants are rare in the literature.

Soil enzyme activity is important in constructed wetlands and sensitive to changes in soil (micro-environmental) conditions such as temperature, pH, plant exudates and soil water chemistry (Shackle et al., 2000). The objective of this study was to investigate the effects of DBP on soil enzyme activity and the degradation of DBP using constructed wetlands soils.