Fast and Slow Phosphate Sorption by Goethite-Rich Natural Materials

Abstract

Abstract--Although phosphate sorption by goethite and other less-abundant Fe oxides strongly influences the concentration of this anion in the soil solution and aquatic environments, relatively little is known on the P-sorption characteristics of natural goethites. For this reason, we examined the P-sorption capacity and time course ofP sorption of 10 goethite-rich soil, ferricrete and lake ore samples, in which the content and nature of mineral impurities were unlikely to affect P sorption significantly. Phosphate sorption could be adequately described by a modified Freundlich equation including a time term. The amount of P sorbed after i day of equilibration at a concentration of 1 mg P/liter ranged widely (0.36-2.04 #mol P/m2). The total P sorbed after 75 days of equilibration varied less, in relative terms (1.62-3.18 #tool p/m2), i.e., a higher slow sorption tended to compensate for a lower initial (fast) sorption. Total sorbed P (X = 2.62, SD = 0.52 ~mol P/m 2) was similar to the sorption capacity of synthetic goethites, suggesting a common sorption mechanism and the predominance of one type of crystal face, which, according to previous transmission electron microscope observations, might be the (110).The extent of the slow reaction correlated to the ratio between micropore surface area and total surface area, as well as to oxalate-extractable Fe, which is an estimation of the ferrihydrite content. Ferrihydrite impurities might affect the slow reaction by contributing to the microporosity of some samples. Silicate adsorbed on the surface of the goethites was readily desorbed during phosphate sorption and did not significantly affect the extent of the slow sorption process.

Chemical Distribution of Phosphorus in Soils used during the Development of Sorption Isotherms

“…Calcium, Al, Fe, and silicate or oxyhydroxide clays are well known for their capacity to sorb P ions from a solution; however, there is a range in the affinity and strength of the bond, depending on where the P is binding (Castro and Torrent 1998;Weng et al, 2012). For example, Ca-bound P, as well as Fe-and Al-bound P can form insoluble precipitates, whereas P bound to clay surfaces is readily exchangeable (Freeman and Rowell 1981;Torrent et al, 1992;de Mesquita Filho and Torrent 1993). The type of clay can also have a dramatic effect on the rate, strength, and total amount of P sorbed; for example, kaolinite can sorb 53.5 times more phosphate than montmorillonite and illite (Shang et al, 2013;Gérard, 2016).…”

“…where Q and c are the same as previously described and k and b are coefficients that are unique to each soil (Sparks 2003;Havlin et al, 2005). The parameter k has been used as a measure of the sorption strength between soil particles and P (Sparks 2003).…”

“…The parameter k has been used as a measure of the sorption strength between soil particles and P (Sparks 2003). The Langmuir equation is used more often than the Freundlich and is described as:…”

“…where Q, c, and k are the same as previously described and b is the sorption maximum or the maximum amount of P that can adsorb to a given soil in units of mg P kg -1 soil (Olsen and Watanabe 1957;Sparks 2003;Havlin et al, 2005). Although Eq.…”

Modelling the geochemical fate and transport of wastewater-derived phosphorus in contrasting groundwater systems

“…This is reflected in typical adsorption coefficients for P on Fe oxides (goethite) that are three orders of magnitude higher than those for calcite (K = 3000 mL g − 1 and 10 mL g − 1 , respectively; Krom and Berner, 1980). Sorption of P is generally considered to be a two-step process; an initial fast adsorption at high affinity mineral surface sites (Parfitt, 1978(Parfitt, , 1989, followed by slow diffusion into micropores or aggregates (Barrow, 1985;Torrent et al, 1992;Slomp et al, 1998;Mikutta et al, 2006) or precipitation of metal phosphate phases (Van Riemsdijk et al, 1984). Phosphate has the ability to form a number of sparingly soluble secondary minerals of which the most common are strengite (FePO 4 · 2H 2 O), vivianite (Fe 3 (PO 4 ) 2 · 8H 2 O), variscite (AlPO 4 · 2H 2 O) and hydroxyapatite (Ca 5 (PO 4 ) 3 OH) (Nriagu and Dell, 1974;Nriagu and Moore, 1984;Zanini et al, 1998;Robertson, 2003).…”