Efficient removal of Congo red from aqueous solutions using phytogenic aluminum sulfate nano coagulant

Aluminum sulfate nanoparticles (ASNPs) were synthesized using Hemigraphis colorata leaf extract with minimal use of corrosive solvents. The material was characterized by Fourier Transform-infrared spectroscopy (FT-IR), X-ray diffraction studies (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDAX), high resolution transmission electron microscopy (HR-TEM), and zeta potential analysis. We were able to suggest a probable mechanism for the formation of ASNPs. Coagulation efficiency of ASNPs for Congo red dye solution was explored under various constraints like coagulant dosage, initial concentration, pH, temperature, presence of inorganic ions and settling time. 140 mgL−1 of ASNPs removed 99.5% of Congo red dye (100 mgL−1) from the solution. The variation in pH of the solution in the range 3–9 and the increase in temperature from 30 °C to 70 °C did not affect the efficiency of the nanocoagulant. It was observed that the generated sludge settled within 30 min and SVI was found to be 31 mLg−1.The nano coagulant can be reused up to five cycles of coagulation. Low dosage, consistent efficiency, easy sludge management, reusability along with cost-effectiveness makes phytogenic ASNPs a potential coagulant for removal of Congo red. •ASNPs of ~13 nm was synthesized using aq. leaf extract of Hemigraphis colorata.•Most probable mechanism is suggested for the formation of ASNPs.•ASNPs is an efficient nano coagulant for Congo red dye from aqueous solutions.•99.5% efficiency with140 mgL−1 for 100 mgL−1dye solution of pH7 at RT.•Efficiency of ASNPs remains unaltered with pH, temperature, ionic interferences.