Sorption and desorption of dissolved organic phosphorus onto iron (oxyhydr)oxides in seawater

Abstract

Sorption of phosphorus (P) onto particulate surfaces significantly influences dissolved P concentrations in aquatic environments. We present results of a study contrasting the sorption behavior of several dissolved organic phosphorus (DOP) compounds and phosphate onto three commonly occurring iron (oxyhydr)oxides (Fe_ox): ferrihydrite, goethite, and hematite. The DOP compounds were chosen to represent a range of molecular weights and structures, and include: adenosine triphosphate (ATP), adenosine monophosphate (AMP), glucose-6-phosphate (G6P), and aminoethylphosphonic acid (AEP).

All P compounds displayed decreasing sorption as a function of crystallinity of the Fe_ox substrate, with ferrihydrite adsorbing the most, hematite the least. In general, maximum sorption density decreased with increasing molecular weight of P compound; sorption of G6P onto goethite and hematite excepted. P compound size and structure, and the nature of the Fe_ox substrate all appear to play a role dictating relative sorption capacity. Failure of a simple, 1-step sorption–desorption model to describe the data suggests that P sorption cannot be explained by a simple balance between sorption and desorption. Instead, the data are consistent with a 2-step sorption model consisting of an initial rapid surface sorption, followed by a slow, solid-state diffusion of P from surface sites into particle interiors. Desorption experiments provide additional support for the 2-step sorption model.

Without exception, DOP compounds showed less efficient sorption than did orthophosphate. This suggests that in aquatic systems enriched in reactive Fe_ox, whether as suspended particulates in the water column or in benthic sediments, DOP bioavailability may exceed that of orthophosphate. Since biological uptake of P from DOP requires enzymatic cleavage of orthophosphate, a system enriched in DOP relative to orthophosphate may impact ecosystem community structure.

Introduction

Phosphorus (P) is a key macronutrient necessary to all living organisms. In aquatic environments P is taken up in dissolved form by primary producers almost exclusively as free orthophosphate (hereafter, phosphate), and incorporated into compounds that make up tissues (apatite in bones and teeth, phospholipids in cell membranes), carry and store genetic information (deoxyribonucleic acid (DNA), ribonucleic acid (RNA)), and store energy (adenosine triphosphate (ATP)). Understanding the processes that control P bioavailability is essential to understanding biological production in aquatic ecosystems. While directly bioavailable as phosphate, Dissolved Organic P (DOP) is rendered bioavailable only after enzyme hydrolysis cleaves phosphate from DOP (e.g., Cembella et al., 1984).

Phosphorus is a highly particle reactive element (e.g., Barrow, 1978, Bolan et al., 1985, Fox, 1990, Khare et al., 2004, and many others). Sorption and desorption of P compounds can exert significant influence on dissolved P concentrations, and thus P bioavailability. Iron (oxyhydr)oxides (hereafter, Fe_ox) have a particularly high capacity to sorb P (e.g., Strauss et al., 1997, Torrent et al., 1992, Khare et al., 2004). Fe_ox in suspended particulate matter or bottom sediments can remove P from natural waters via sorption, rendering it unavailable for biological uptake. Reductive dissolution of Fe_ox in suboxic or anoxic environments will liberate associated P to solution (e.g., Froelich et al., 1979, Burdige, 2006, Slomp et al., 1996a). Desorption from particle surfaces is another avenue for sorbed P compounds to enter the solution phase, and once again become available to primary producers. Thus, Fe_ox can act as either a source or a sink of P in aquatic systems (e.g. Froelich, 1988, Fox, 1990, Slomp et al., 1996a, Slomp et al., 1996b, Chitraker et al., 2006).

Numerous studies have examined sorption of phosphate onto marine and freshwater sediments (e.g., Rodel et al., 1977, Slomp et al., 1996a, Slomp et al., 1996b, Slomp et al., 1998, Sundareshwar and Morris, 1999) and Fe_ox (e.g., Bolan et al., 1985, Parfitt, 1989, Strauss et al., 1997). Sorption and desorption processes are believed to buffer phosphate concentration in rivers (Mayer and Gloss, 1980, Froelich, 1988) and estuaries (Pomeroy et al., 1965, Fox et al., 1985). There is a voluminous soil science literature on phosphate sorption onto soils and soil minerals (e.g., Frossard et al., 1995, McGechan and Lewis, 2002). Fewer studies have explored dissolved organic P (DOP) sorption, and these have focused exclusively on terrestrial soil (e.g., Frossard et al., 1995, Leytem et al., 2002, Berg and Joern, 2006) or freshwater (Rodel et al., 1977) systems. While studies of P sorption onto soils can be instructive when considering similar processes in marine systems, they are not directly relevant because of the profoundly different ionic strength and pH of soils versus marine systems. We are unaware of any studies that examine DOP sorption or desorption onto Fe_ox in marine systems.

The 2-step model for phosphate sorption onto mineral surfaces, widely accepted by the soil science community, describes the process of phosphate sorption onto Fe_ox as consisting of a fast initial uptake onto mineral surfaces, taking place over the course of minutes, followed by a slow reaction, lasting for days or even months (Barrow, 1978, Strauss et al., 1997, McGechan and Lewis, 2002, Luengo et al., 2006). The fast reaction is dominated by ligand exchange in which hydroxyl groups or water molecules on (oxyhydr)oxide surfaces are replaced by ${\text{PO}}_4^{3-}$ anions (Fig 1, Region A) (Parfitt, 1978, Torrent et al., 1992). This first reaction, step 1 of the 2-step process, is assumed to be reversible, allowing for the release of sorbed P to solution if environmental conditions favor desorption (McGechan and Lewis, 2002). Step 1 continues in parallel with step 2 as long as conditions favor P sorption. The slower reaction, step 2, represents solid-state diffusion of phosphate into the interior of Fe phases (Fig 1, Region B) (Barrow, 1983, Torrent et al., 1992, McGechan and Lewis, 2002). This second reaction, induced in response to the concentration gradient set up by the initial sorption that occurred in step 1, is largely irreversible (however, see Lookman et al., 1995). The amount of sorbed P available for desorption is therefore influenced by the length of time that Fe_ox is exposed to dissolved P, in an environment in which sorption is favorable, prior to being exposed to conditions that favor desorption (Munns and Fox, 1976, Froelich, 1988).

Froelich (1988) introduced the 2-step model to the aquatic geochemistry community, and several studies of marine systems have observed phosphate sorption behavior that is empirically consistent with it (e.g., Millero et al., 2001, McGlathery et al., 1994, Vidal, 1994). There is emerging consensus that this model is appropriate for marine systems, but the rigorous evaluation that has been carried out for soils has yet to be undertaken for marine systems. We are unaware of any studies that examine whether the 2-step model applies to sorption of DOP.

This study reports on experiments that examine the sorption and desorption characteristics of several DOP compounds onto/off of Fe_ox in artificial seawater; parallel experiments were run using phosphate as a control compound. Equilibrium conditions of sorption reactions and kinetic uptake of P compounds were examined. Desorption experiments were executed after both short- and long-term pre-sorption treatments.