Phosphorus adsorption on natural sediments: Modeling and effects of pH and sediment composition
Introduction
As one of the main nutrients, P can be found in the sediment matrix in the forms of calcium, iron or aluminum complex salts and organic species, or adsorbed onto the surface of minerals and has a predominant role in cellular energetics as ATP and as an important part of many structural and biochemically functional components for cell growth of plants including algae (Gibson, 1971; Dorich et al., 1984), but its excess supply can lead to eutrophication. The role of P in the fresh water eutrophication process as the primary limiting factor for algal growth was identified several decades ago (OECD, 1982). Much attention has been paid to P adsorption/desorption processes on natural sediments and pure minerals (Naoml and Patrick, 1991a, Naoml and Patrick, 1991b; Yao and Millero, 1996; Zhao and Stanforth, 2001; Kim et al., 2003). Laboratory experiments with Fe(OOH) showed that pH increases produced liberation of P adsorbed onto the iron complexes because of competition with OH− ions (Lijklema, 1977). But little information about the effects of pH in a wide range on P sorption on natural sediments is available.
NAP content and sorption characteristics have been correlated with sediment composition such as active iron, aluminum, and organic matter content, and in turn, sediment composition has been related with lake chemistry, i.e., with hardness, alkalinity, and pH (Naoml and Patrick, 1991a). This fraction of P will certainly participate the adsorption/desorption processes in batch experiments conducted in laboratory. Especially, when NAP is significant, it cannot be neglected in the descriptive model for adsorption/desorption processes.
In the past, researchers had made much effort to develop models to describe P adsorption characteristics on natural sorbents. For instance, Hiemstra and coworkers have developed CD-MUSIC model to describe the sorption behaviors of P on such pure minerals as goethite (Hiemstra and Van Riemsdijk, 1996; Geelhoed et al., 1998), but this model has difficulty in being applied to heterogeneous sorbents. So far, simple Langmuir isotherm model is still extensively used to describe sorption features of P on natural sediments (Lopez et al., 1996; Bubba et al., 2003). However, as far as NAP-polluted sediments are concerned, the simple Langmuir model cannot work perfectly because it neglected the existence of native adsorbed P (House and Denison, 2000).
The objectives of present study are mainly to model P adsorption processes by modified Langmuir isotherm and understand the influencing patterns of such factors as pH, zeta potential, ORP, and sediment composition.
Section snippets
Theory
When sediment contacts with water, P will be exchanged with the water on the interface until a dynamic equilibrium is reached. Sediment and water concentrations are stabilized when sediment-to-water and water-to-sediment rates become equal. Adsorption–desorption and precipitation–dissolution may be involved in these processes. The equilibrium situation is usually described by a Langmuir adsorption isotherm (Stumm and Morgan, 1981; Weber et al., 1991).where NSP is the mass of
Description of Taihu Lake area
Taihu lake, located in south of the Changjiang delta, China, is a large, shallow and eutrophic lake in the Yangtze valley, 90 km from Shanghai and subjects to severe blooms of cyanobacteria. It has an area of 2338 km2 and a mean depth of 2 m. Trophic states in different areas of the lake vary from hypereutrophic to mesotrophic states related to different extents and manners of anthropogenic activities. It is of great importance for drinking water source of cities like Wuxi, Jiangsu province, and
Sediment characteristics and elemental content
Chemical component concentrations measured in the lake sediment samples used for sorption experiments are summarized in Table 2. Organic matter, expressed as LOI, exhibits concentrations ranging from 4.51% dw (site T6) to 10.20% dw (site T1) in Taihu Lake. The oxidizable matter (TOC) exhibits the lowest content (0.66% dw) in the center of Taihu Lake (site T4) and the highest content (1.97% dw) in the site of T1. T1 has the highest value of LOI and TOC, which may relate to the fact that T1 is
Conclusion
Although the sedimentary P cycle has been the topic of many previous studies, the dynamics, controls and effects of P sorption in natural sediments are still incompletely understood. Considering NAP the modified Langmuir equation interprets well the sorption data of P on natural sediments, especially when NAP is significant like site T1. When the modified equation could be properly applied, relevant parameters PAC, NAP, EPC0 and k were calculated. PAC and partitioning coefficient Kp were
Acknowledgements
The study was financially supported by the important project of the knowledge innovation program of Chinese Academy of Sciences under KZCX1-SW-12, (2002–2006), and NSF of China under 20037010.
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