Ionic Strength Effects on Sulfate and Phosphate Adsorption on γ‐Alumina and Kaolinite: Triple‐Layer Model

Abstract

It is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral‐water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on γ‐Al2O3 and kaolinite at varying solution pH (3–11), and to infer SO4 and PO4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO4 on γ‐Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on γ‐Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple‐layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation.

A Discussion on Mehlich-3 Phosphorus Extraction from the Perspective of Governing Chemical Reactions and Phases: Impact of Soil pH

“…Soil organic P as Al, Fe, and Ca-phytate shows similar patterns in solubility with changes in pH, as their inorganic P mineral (and adsorbed phases) counterparts [27]. It is important to note that in addition to precipitated Al and Fe-P minerals, P adsorbed onto the surface of variable charged Al and Fe oxides/hydroxides will also tend to desorb less at low pH due to decreased competition with hydroxide [28]. …”

“…This is due to the fact that Al hydrolyzes rapidly as pH increases above 5, with hydroxyl becoming a better competitor for Al than P [43]. This will also shift the P forms associated with Al from Al-P minerals to P bound onto the surface of Al oxide/hydroxide minerals via ligand exchange [28,44,45]. Consider that for many Al-P minerals, the Al:P molar ratio is less than 2:1 and approaches 1:1 when pH is low enough to prevent extensive Al hydrolysis [45].…”

Effect of KCl and CaCl2 as Background Electrolytes on the Competitive Adsorption of Glyphosate and Phosphate on Goethite

“…Moreover, ionic strength also affects adsorption of phosphate as is observed by comparing the adsorption of phosphate in 0.01 M and 0.1 M KC1 (Figure 1). Barrow et aL (1980), Bolan et al (1986) and He et al (1997) found that there is a pH value below which phosphate adsorption decreases with increasing ionic strength and above which the adsorption increases with increasing ionic strength. This pH value was found to be -4.5 for phosphate adsorption on goethite (Barrow, 1985).…”

“…This result indicates that the adsorption of glyphosate on goethite is different from the adsorption of phosphate. Bolan et al (1986) and He et al (1997) found that the adsorption of phosphate and sulfate was not affected in the same way by a change in ionic strength, and they concluded that this was related to differences in the adsorption mechanisms.…”

Adhesion of Saccharomyces cerevisiae to Alumina Surfaces and Its Removal by Caustic Alkali Cleaning.

“…On the other hand, short-range forces involved in the irreversible adhesion process are thought to be dipole-dipole interactions, hydrogen, ionic, and covalent bonding, and hydrophobic interactions (Kumar and Anand, 1998). It has been reported that the phosphate ion and carboxylic acids were adsorbed on solid mineral surfaces (Al2O3, kaolinite, and FeOOH) by forming inner sphere complexes (Filius et al, 1997;He et al, 1997). It is likely that the phosphate groups and side-chain carboxyl groups on cell wall might play an important role in the irreversible adsorption process through non-electrostaic interaction, e.g., coordinate (covalent) bonding.…”