Novel approach for selective phosphate removal using colloidal layered double hydroxide nanosheets and use of residue as fertilizer

Onsite remediation of phosphate in wastewaters remains a challenge in water treatment technologies. Further, aftermath of the treated material is hitherto unknown and mostly disposed of as land-fill. A new method for onsite remediation of phosphate is proposed using colloidal layered double hydroxide (LDH) nanosheets and the treated material is valorized as phosphate release fertilizer. Nitrate containing NiAl-LDHs was prepared by hexamine hydrolysis and the colloidal suspension of LDH nanosheets was obtained by dispersing the wet cake in water through three different methods namely ultrasonication, mechanical shaking and hydrothermal. These nanosheets were explored for phosphate remediation; the quantity of phosphate removed depended on the amount of LDH present in the colloidal suspension. Powder X-ray diffraction, TEM, and UV–vis diffuse reflectance spectroscopic measurements revealed that the uptake is due to restacking of nanosheets. The uptake of phosphate was rapid for UD-LDH compared to parent powdered LDH; however, settling of nanosheets was hindered at higher than the stoichiometric amount of UD-LDH and is reasoned through zeta potential measurements. Uptake of phosphate was similar in wide pH range (4.5–11) while decreased sharply at lower pH. Selective uptake of phosphate in enriched seawater and sustained removal of phosphate in simulated “model pond systems” were achieved for UD-LDH. The restacked material after phosphate removal was utilized as an effective phosphate release fertilizer for the growth of a common green seaweed Ulva lactuca. Tapping the potential of colloidal LDH nanosheets for wastewater remediation and ingeniously using the treated material, here as fertilizer, offers a new approach with dual benefits.