The role of Al- and Fe-oxy-hydroxides in determining surface properties of soil ped models, with emphasis on phosphorus sorption/desorption phenomena

An investigation was carried out to elucidate the role of Al or Fe poorly-ordered oxyhydroxides in determining the surface properties of soil ped models formed by interaction with bentonite clay (C) and polyphenol (pp) in acidic milieu, with emphasis on phosphorus (P) sorption/desorption phenomena. P was chosen because of its important involvement on pedogenic processes as well as on soil fertility. P was added to models (Pa) at the rate of 10 mg·g-1. Pa was then extracted from the models at different aggregation states (suspended, freeze dried, air-dried, re-suspended), by using the electro-ultrafiltration technique (EUF-P). The cumulative amount EUF-P extracted after 40 min, representing the loosely-bound immediately-exchangeable P (Pi) and the maximum desorbable EUF-P, representing the slowly released P (Pm) were considered. The models were analyzed for their Cation Exchange Capacity (CEC), Specific Surface Area (SSA), Specific Cationic Surface Charge (SCSC), Titratable Acidity (TA) to pH = 7.0 or pH = 8.0 by KOH or by Ca(OH)2, and Phosphorus Sorption Coefficient (PSC). The C-Fe-pp model was characterized by an elevated surface reactivity, showing values of CEC (1777 μeq·g-1), SSA (483.2 m2·g-1), SCSC (3.678 μeq·m-2), TA (from 572 to 804 μeq·g-1) and PSC (92.4%), which were considerably larger than the respective determined for the C-Al-pp model (CEC= 1173 μeq·g-1, SSA = 420.0 m2·g-1, SCSC = 2.793 μeq·m-2, TA from 36 to 176 μeq·g-1, and PSC = 76.8%). Contrastingly, both the amounts EUF Pi and Pm desorbable from the same C-Fe-pp model were much larger than the respective determined for the C-Al-pp model. With reference to the aggregation state of the prepared materials, Pi decreased on the average as suspended > freeze-dried > air-dried ≥ resuspended. The desiccation via air-drying definitively stabilized the P pool, especially in the C-Al-pp model. Such stabilization was irreversible, since both Pi and Pm for C-Al-pp, and Pi for C-Fe-pp, did not substantially change in the re-suspended materials. With regard to kinetics aspects, the EUF-P desorption from C-Al-pp always followed a 2nd reaction order, while that from C-Fe-pp followed zero or 1st reaction order from suspended and freeze-dried, or from air-dried and re-suspended materials, respectively. The achieved data clearly demonstrated that the different interactions of bentonite clay and polyphenol with Al- or Fehydrolytic products led to the formation of aggregates with peculiar surface properties and physical-chemical behavior, with distinct mechanisms controlling the interactions of P with C-Al-pp or C-Fe-pp in both sorption and desorption phenomena.An investigation was carried out to elucidate the role of Al or Fe poorly-ordered oxyhydroxides in determining the surface properties of soil ped models formed by interaction with bentonite clay (C) and polyphenol (pp) in acidic milieu, with emphasis on phosphorus (P) sorption/desorption phenomena. P was chosen because of its important involvement on pedogenic processes as well as on soil fertility. P was added to models (Pa) at the rate of 10 mg·g-1. Pa was then extracted from the models at different aggregation states (suspended, freeze dried, air-dried, re-suspended), by using the electro-ultrafiltration technique (EUF-P). The cumulative amount EUF-P extracted after 40 min, representing the loosely-bound immediately-exchangeable P (Pi) and the maximum desorbable EUF-P, representing the slowly released P (Pm) were considered. The models were analyzed for their Cation Exchange Capacity (CEC), Specific Surface Area (SSA), Specific Cationic Surface Charge (SCSC), Titratable Acidity (TA) to pH = 7.0 or pH = 8.0 by KOH or by Ca(OH)2, and Phosphorus Sorption Coefficient (PSC). The C-Fe-pp model was characterized by an elevated surface reactivity, showing values of CEC (1777 μeq·g-1), SSA (483.2 m2·g-1), SCSC (3.678 μeq·m-2), TA (from 572 to 804 μeq·g-1) and PSC (92.4%), which were considerably larger than the respective determined for the C-Al-pp model (CEC= 1173 μeq·g-1, SSA = 420.0 m2·g-1, SCSC = 2.793 μeq·m-2, TA from 36 to 176 μeq·g-1, and PSC = 76.8%). Contrastingly, both the amounts EUF Pi and Pm desorbable from the same C-Fe-pp model were much larger than the respective determined for the C-Al-pp model. With reference to the aggregation state of the prepared materials, Pi decreased on the average as suspended > freeze-dried > air-dried ≥ resuspended. The desiccation via air-drying definitively stabilized the P pool, especially in the C-Al-pp model. Such stabilization was irreversible, since both Pi and Pm for C-Al-pp, and Pi for C-Fe-pp, did not substantially change in the re-suspended materials. With regard to kinetics aspects, the EUF-P desorption from C-Al-pp always followed a 2nd reaction order, while that from C-Fe-pp followed zero or 1st reaction order from suspended and freeze-dried, or from air-dried and re-suspended materials, respectively. The achieved data clearly demonstrated that the different interactions of bentonite clay and polyphenol with Al- or Fehydrolytic products led to the formation of aggregates with peculiar surface properties and physical-chemical behavior, with distinct mechanisms controlling the interactions of P with C-Al-pp or C-Fe-pp in both sorption and desorption phenomena