Environ. Sci. Technol. 2007, 41, 2789-2795
Relative Importance of Solid-Phase Phosphorus and Iron on the Sorption Behavior of Sediments J I A - Z H O N G Z H A N G * ,† A N D X I A O - L A N H U A N G †,‡ Ocean Chemistry Division, Atlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administration, Miami, Florida 33149, and CIMAS, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149
Of all the metal oxide particles, amorphous iron oxides have the greatest adsorption capacity for phosphate. Coastal sediments are often coated with terrigenous amorphous iron oxides, and those containing high iron are thought to have a high adsorption capacity. However, this conventional wisdom is based largely upon studies of phosphate adsorption on laboratory-synthesized minerals themselves containing no phosphorus. Using natural sediments that contain variable phosphorus and iron, our results demonstrate that the exchangeable phosphate rather than the iron oxides of sediments governs the overall sorption behavior. The iron oxide content becomes important only in sediments that are poor in phosphorus. A total of 40 sampling sites across the Florida Bay provide detailed spatial distributions both of the sediment’s zero equilibrium phosphate concentration (EPC0) and of the distribution coefficient (Kd) that are consistent with the distribution of the exchangeable phosphate content of the sediment. This study provides the first quantitative relationships between sorption characteristics (EPC0 and Kd) and the exchangeable phosphate content of natural sediments.
Introduction Although phosphorus (P) is an essential macronutrient to living organisms, its biological availability in aquatic environments is limited by the low solubility of P-bearing minerals and the strong affinity of dissolved phosphate to solid surfaces. For example, more than 90% of the riverine flux of P to the oceans is associated with suspended particulate matter (1). Most riverborne suspended particulate matter is aggregated and settles out, becoming bottom sediments in estuarine and coastal regions (2). Not surprisingly, sediment has been identified as the dominant P reservoir in a variety of aquatic ecosystems (3-6). As a result, P exchange across the sediment-water interface, through adsorption-desorption and coprecipitation-dissolution processes, plays a critical role in governing dissolved phosphate concentrations in the overlying waters (7, 8). Such exchange processes are largely controlled by the chemical composition and physical properties of particles. Among the different metal oxide particles, amorphous iron oxides have been shown to have the greatest adsorption capacity for phosphate (9-14). * Corresponding author phone: (305) 361-4512; fax: (305) 3614447; e-mail:
[email protected]. † AOML, NOAA. ‡ CIMAS, University of Miami. 10.1021/es061836q CCC: $37.00 Published on Web 03/16/2007
2007 American Chemical Society
Because iron is ubiquitous in the environment, the content of amorphous iron oxides in the sediments has been considered to be the most important factor in regulating sediment’s adsorption capacity. Coastal sediments are often coated with terrigenous amorphous iron oxides, and those containing high iron are thought to have a high adsorption capacity. However, this conventional wisdom is based largely upon studies of phosphate adsorption on laboratorysynthesized minerals themselves containing no P (9-12). Natural particles, such as sediments, soils, and dust, always contain P, an abundance of which depends upon their origin and sedimentary environments. Although aquatic science literature is replete with phosphate sorption studies, studies on sediment sorption in conjunction with the analysis of solid-phase P speciation have been rare. Moreover, due to a heavy workload involved in sediment sampling, processing, and analysis, most studies are often limited to a few sampling sites, in many cases, sampling only two end-members. A meaningful spatial distribution pattern cannot be obtained, and a statistically sound relationship, particularly a nonlinear function, cannot be derived from such studies. To our knowledge, the influence of solid-phase P on phosphate sorption behavior of sediments has not been systematically studied. The processes of P exchange across the sediment-water interface become particularly important in shallow coastal systems such as the Florida Bay where dissolved phosphate in the water column is present at nanomolar concentrations (15, 16) and P has been identified as a limiting nutrient to seagrass, phytoplankton, and bacteria (17-19). Sediments in the Florida Bay consist mainly of biogenic carbonate (81-96% by weight (20)), which strongly adsorbs phosphate (21-23). The amount of adsorption is known to be proportional to surface area, which increases exponentially with a decreasing particle size. It has been shown that fine particles accounted for only 16% of weight but represented 76% of total surface area in bulk sediments (24). In addition, fine particles are easy to resuspend into the water column by disturbances. Once in the water column, their residence times are much longer than coarser particles since their settling velocity is proportional to the square of the particle diameter (25). In the shallow waters of the Florida Bay (average water depth 1 m), fine-grained carbonate muds were readily resuspended to the surface by wind and tidal mixing and supply P to euphotic zone for biological assimilation (26). Wind conditions sufficient to resuspend fine-grained sediments throughout the Florida Bay occur at an ca. weekly interval of frontal passages during the dry season, while localized strong convective events are common during the wet season (27). Accurately quantifying the P exchange across the sediment-water interface during sediment resuspension is essential for estimating benthic P flux and understanding the biogeochemistry and ecology of the bay. Our previous study documented a strong spatial gradient in both sedimentary P and iron across the bay (20). It is reasonable to hypothesize that sorption behavior of sediments will systematically vary due to the gradients in these physicochemical properties. The aim of the present study is to document the spatial variability of sediment characteristics with respect to the sediment-water exchange of P and to identify the factors governing such variability.
Experimental Procedures Study Region. Located at the southern end of the Florida peninsula, the Florida Bay is one of the world’s largest (2200 VOL. 41, NO. 8, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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tration, EPC. The difference between [P]i, and [P]f was used to calculate the amount of phosphate adsorbed (if positive) to or desorbed (if negative) from the sediment, ∆[Psed]
∆[Psed] ) [P]i - [P]f
(1)
A plot of ∆[Psed] versus [P]f at a given temperature is called an adsorption isotherm. A modified Freundlich equation was used to parameterize the adsorption isotherm data
∆[Psed] + NAP ) Kf([P]f)n
FIGURE 1. Location of the 40 sediment sampling sites across the Florida Bay. km2) coastal lagoons (Figure 1). Its triangular-shaped area is bordered to the north by the Everglades, the world’s largest wetlands. A chain of islands known as the Florida Keys separates the shallow bay from the Atlantic Ocean, forming its eastern and southern boundaries. Its westerly margin is open to the Gulf of Mexico, although water exchange is highly restricted by extensive shallow western margin mudbanks. Dotted mangrove islands and a complex network of carbonate mudbanks divide the interior bay into numerous isolated sub-basins with maximal water depths of 2-3 meters. Water exchange between sub-basins occurs through narrow cuts and overbank wash. Our previous study revealed that the external sources of two limiting nutrients, P and iron, to the Florida Bay were spatially separated with P introduced by coastal waters across its western margin and iron introduced from freshwater canal flows into the eastern bay (20). Sediment Samples. Surface sediments were collected from 40 stations across Florida Bay (Figure 1). Details of sampling and sample processing were given elsewhere (20). To simulate resuspension events, easily resuspended finegrained sediments (
