Alkaline fly ash effects on boron sorption and desorption in soils

Application of alkaline fly ash to soils is expected to result in an increase in B sorption capacity. If fly ash is B-rich, B phytotoxicity might occur depending not only on B loads and magnitude of soil sorption capacity for B, but also on the strength of B retention by sorption surfaces of the fly ash amended soils. This strength determines the ease through which B releases into the soil solution. Aged-alkaline fly ash was applied to one calcareous and two acid soils at rates equal to 0, 5, 20, and 50 g kg(-1) of soil, and the impact of fly ash addition on B sorption in these soils was characterized, by means of the parameters (affinity and maximum) obtained through fitting B sorption data to the nonlinear Freundlich, Langmuir isotherms, and the phenomenological equation of Keren et al. Boron was added to the untreated and the fly ash-treated soils, left in contact for 30 d, and its desorbability was studied. It was observed that although B sorption maximum of soils tended to increase upon fly ash addition, the affinity of B to sorption sites remained practically unaltered in most of the cases. Boron sorption was an exothermic reaction and the greatest part (more than 60%) of sorbed B in the fly ash-treated soils could be easily desorbed within 24 h, reaching 80% for the acid soils and 100% for the calcareous soil after 120 h of desorption time. It was concluded that although there was a tendency of an increase in B sorption capacity in most cases upon fly ash addition, this increase was not generally accompanied by an increase in strength of B retention by soil surfaces. A major part of added B in the fly ash-treated soils remained labile enough to be released in the soil solution in a short time.