CHARACTERIZING SORPTION-DESORPTION AND TRANSFORMATION OF PHOSPHORUS IN ACID SOILS USING NANO COMPOSITE SORBENT OF Fe-Al-Mn MIXED OXIDE: AN APPROACH FOR PREDICTING PLANT AVAILABLE PHOSPHORUS

Abstract

Phosphorus (P) fixation in acid soils is the major cause of P deficiency which necessitates application of extra fertilizer P even while the total soil P capital is large. As cure to this problem, understanding of P release kinetics and transformation is very important. One approach to get this understanding is to use nanocomposite sorbent of ternary metal oxides that are used to study long-term desorption and transformations of P. Therefore, this work aimed at synthesizing and applying suitable mixed metal oxide nanocomposite sorbents to characterize long term P desorption kinetics, studying transformation of P in different P pools, and investigating liming effect on P desorption and plant availability in incubated acid soils. Nanostructured Fe–Al–Mn ternary metal oxide sorbents of different molar ratios were prepared using simultaneous oxidation and co-precipitation method. The adsorbents were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Brunauer, Emmett and Teller (BET) and Fourier transform infrared spectroscope (FT-IR). The results indicated that the Fe– Al–Mn ternary metal oxides were amorphous and nanostructured. The 3:3:1 molar ratio of Fe:Al:Mn produced nanocomposite sorbent with the highest surface area and was selected for subsequent application. The adsorption isotherms on the adsorbent were described by Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin models at pH 6.5. Desorption kinetics of applied P to four acid clay soils were investigated over 56 days using the Fe-Al-Mn ternary metal oxide in dialysis membrane tubes (DMT-HFAMO) as P sink. The four acid clay soils were collected from four districts: Boji Dirmaji (BD), Nedjo (N), Kiltu Karra (KK) and Mene Sibu (MS). The soil treatments followed completely randomized design (CRD) in which four rates of P and lime were applied in three replicates of the four samples. After 112 days of incubation, P desorption was studied for 56 days. Phosphorus desorption kinetics was described with a two-component second-order model. Two discrete P “pools” were assumed: a labile P pool (SPA) and less labile P pool (SPB). P fractionation was carried out to determine transformation and distributions of P in the various P pools. Effects of lime and mineral P fertilizers on growth of maize (Zea mays L.) and P uptake were investigated in pot experiment. Phosphate adsorption onto the sorbent xviii gradually decreased with increasing pH. The adsorption isotherms followed the fitting order: Freundlich>Temkin> Dubinin-Radushkevich >Langmuir. At 35 °C, the maximum adsorption capacity for the adsorbent was about 49.95 mg/g, which was higher than their reported single metal oxide. The coexisting anions competed with phosphate to be adsorbed on the adsorbent in the order SiO3 2- > CO3 2–> SO4 2– > NO3 -, which is closely related to charge-to-radius ratios of the anions. Desorption maximum was not reached in all of the treated soils within the 56 days. The lime requirements to raise pH to target values of 5.5, 6.5, and 7.2 varied from 4.27 to 8.18 tons CaCO3 ha-1. The total P contents of the studied soils ranged from 298.46 to 392.12 mg kg-1. Available Bray I-P ranged from 1.12 to 1.82 mg kg-1 and was rated as very low available P content. On average, distributions of P in the various pools followed the order: iron bound P (Fe-P) > aluminum bound P (Al-P) > calcium bound P (Ca-P) > easily soluble P (ES-P) > reductant soluble P (RS-P). Desorption of P from the various fractions were differently affected by clay content, CEC and pH of the soils. The recovered mean percentages of the applied P were different among the soil types and P treatments and ranged from 44.30% to 90.83%. Interaction of P lime brought about relatively larger increment in P availability. Plant biomasses of the maize increased with rates of P and lime. Generally, this work demonstrated that long-term P desorption study in acid soils can be effectively characterized using DMT-HFAMO to understand P desorption kinetics and transformation, and effects of applied P and lime on P desorption and to predict plant available P.