Sorption of nutrients (orthophosphate, nitrate III and V) in an equimolar mixture of P–PO4, N–NO2 and N–NO3 using chitosan

This manuscript presents results of a study on sorption effectiveness of orthophosphate and nitrate III and V in an equimolar mixture of P–PO4, N–NO2 and N–NO3 using chitosan in the form of hydrogel beads. Analyses were carried out for: the effect of pH on sorption intensity of orthophosphate and of nitrate III and V onto chitosan, sorption kinetics onto chitosan, and the maximum sorption capacity of P–PO4, N–NO2 and N–NO3 by chitosan in a mixture.The most effective sorption of P–PO4, N–NO2 and N–NO3 was observed at pH 4. In turn, at pH 6–11 the sorption of orthophosphate and nitrate onto chitosan proved ineffective. P–PO4, N–NO2 and N–NO3 sorption kinetics was best described by the pseudo-second order model. The model of intraparticle diffusion fitting to experimental data demonstrated that the process of orthophosphate and nitrate sorption onto chitosan proceeded in 2 phases, i.e. 1 – adsorption and 2 – absorption.A phenomenon of the partial release of P–PO4, N–NO2 and N–NO3 after a specified period of time was observed in the study, which was due to a change in solution pH induced by the sorbent. The period after which desorption occurred was 120 min in the case of phosphates and 60 min in the case of nitrates III and V. The quantity of bound sorbate (a mixture of orthophosphate and nitrate) was the highest in the 60th minute of sorption.The study demonstrated evident competition between H2PO4−, NO2− and NO3− ions. The affinity of orthophosphate and nitrate to the sorption centers of chitosan could be ordered as follows: P–PO4 > N–NO2 > N–NO3. Sorption equilibrium isotherms were best described by a heterogeneous Langmuir’s model. In the equimolar mixture of P–PO4, N–NO2 and N–NO3, the sorption capacity of 1 g of chitosan reached 0.507 mmol H2PO4−/g, 0.373 mmol NO2−/g and 0.350 mmol NO3−/g, which means a total of 1.23 mmol nutrients. Keywords: Sorption, Chitosan, Nitrates III, Nitrates V, Orthophosphates, Kinetics