Potassium Sorption to Aluminum and Silicon Oxides Is Affected by Co-Ions (Magnesium, Zinc, and Nickel), Dissolved Silicate, and Surface Precipitation

Potassium bioavailability in soil is a major problem for efficient crop production, and it is directly linked to the behavior of potassium at the mineral-water interface. Potassium adsorption is known to occur in soil via outer-sphere as well as inner-sphere surface complexation, depending on the soil conditions (e.g., mineralogy). However, the main focus of this study is to identify novel mechanisms of potassium sorption to newly formed surface precipitates. Soil clay minerals and metal oxides can undergo dissolution on a short time scale (e.g., minutes to weeks) at circumneutral pH values and be subjected to surface precipitation. We hypothesize that the formation of surface precipitates can create new adsorption sites for potassium, thus affecting potassium mobility. The objective of this study is to determine how potassium sorption to aluminum and silicon oxides is affected by co-ions, dissolved silicate, and surface precipitation. Potassium sorption experiments were conducted at a pH of 8.5 using AlO(OH) and SiO2 sorbents in the presence of co-ions (magnesium, nickel, and zinc). The soils of the Texas High Plains, where potassium bioavailability is a significant problem, is alkaline (pH 7.8-8.2), and therefore, a relatively alkaline pH was selected for this study. Adsorption results indicate that SiO-2 played an important role in increasing potassium sorption compared to AlO(OH) due to its role in the formation of silicated metal hydroxides. Surface area normalized adsorption values indicate that potassium adsorption increases up to 29% on average when dissolved silicate is present with surface precipitates. X-ray diffraction analysis of the reacted solids indicated that the presence of zinc and nickel compared to magnesium enhanced the formation of new surface precipitates, which were indexed to layered double hydroxides (LDHs). X-ray absorption near edge structure analysis indicated that potassium may be bound to these surface precipitates (e.g., LDHs) via inner-sphere surface complexation. Overall, these results provide novel insights into potassium adsorption and fixation in soil and clay mineral systems, and they have important implications for effective potassium fertilizer application and bioavailability in soil because of the influence that surface precipitates have on cation sorption.