This study reports on the removal of orthophosphate (Pi) by coagulation–flocculation with variable combinations of alum, biopolymers, and biomass. The combinatorial effects of these coagulant aids were evaluated for single, binary, and ternary systems. The role of pH, component dosages, and Pi concentration on the coagulation–flocculation efficacy was evaluated. There was an optimal dosage of alum (30 mg/L) while alginate and chitosan were 15 mg/L. Pi removal was 86% for alum and 98% for ternary systems containing chitosan and alginate where [Pi] = 10–11 mg Pi/L. Pi removal for the alum–alginate–chitosan ternary system was more efficient than that for the binary systems, especially at pH 6–7, where reduced efficiency occurred at pH > 7.5. Pi removal was independent of concentration except at lower levels, [Pi] < 10 mg/L. The alum–refined oat hull binary system was 99% effective for Pi removal, especially when [Pi] = 25 mg/L, with greater removal over the use of oat hulls alone.

A coagulation–flocculation process was employed to remove orthophosphate (Pi) in aqueous media using a ferric chloride (FeCl3) and alginate flocculant system. Jar tests were conducted, and the response surface methodology (RSM) was used to optimize the Pi removal variables. The Box–Behnken design was used to evaluate the effects and interactions of four independent variables: pH, FeCl3 dose, alginate dose, and settling time. The RSM analysis showed that the experimental data followed a quadratic polynomial model with optimum conditions at pH 4.6, [FeCl3] = 12.5 mg·L–1, [alginate] = 7.0 mg·L–1, and a 37 min settling time. Optimum conditions led to a Pi removal of 99.6% according to the RSM optimization, in good agreement with experimental removal (99.7 ± 0.7%), at an initial concentration of 10.0 mg Pi/L. The isotherm adsorption data at the optimized conditions were analyzed by the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models and several isotherms models (Langmuir, Freundlich, and Sips). The PFO kinetic model and Langmuir isotherm model yielded the best fit to the isotherm results. The maximum adsorption capacity of the flocculant system was 83.6 mg·g–1. The flocculation process followed electrostatic charge neutralization and an ion-binding adsorption mechanisms.

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Environmental contextThe fate and build-up of phosphate nutrients in aquatic environments is an urgent environmental problem affecting global water security. This study, guided by a statistical design method, optimises the flocculation properties of a biopolymer for removing orthophosphate from water. This improved technology has potential widespread applications for removal of orthophosphate from water and wastewater treatment systems. AbstractA coagulation-flocculation process was employed to remove turbidity (Ti) and orthophosphate (Pi) in aqueous media using a ferric chloride (FeCl3) and a grafted carboxymethyl chitosan (CMC) flocculant system. The amphoteric CMC-CTA flocculant was synthesised by grafting 3-chloro-2-hydroxypropyl trimethylammonium chloride (CTA) onto the biopolymer backbone of CMC. Here, CMC-CTA denotes the covalent grafting of CTA onto CMC. Optimisation of the variables for Pi and Ti removal was conducted using a jar test system based on the experimental design obtained from the response surface methodology (RSM). The Box–Behnken design was used to evaluate the individual and interactive effects of four independent variables: CMC-CTA dosage, FeCl3 dosage, pH and settling time. The RSM analysis showed that the experimental data followed a quadratic polynomial model with the following optimal conditions: [CMC-CTA]=3.0mgL−1, [FeCl3]=10.0mgL−1, pH 6.8 and settling time=35min. Optimum conditions led to a Pi removal of 96.4% and turbidity removal of 96.7% based on the RSM optimisation, in good agreement with experimental results with an initial concentration of 30.0mg PiL−1. The coagulation-flocculation process is characterised by a combination of electrostatic charge neutralisation, polymer bridging and a polymer adsorption mechanism.