Sustainable iron-grafted cellulose fibers enable coagulant recycling and improve contaminant removal in water treatment

Large amounts of expensive single-use and metal-based coagulants and synthetic flocculants are employed and lost in settled sludge during water treatment. In this work, cellulose fibers were grafted with iron (hydr)oxides and introduced as a dispersed suspension during coagulation/flocculation to simultaneously act as a super bridging agent and adsorbent and significantly improve the treatment process. Fe-grafted fiber concentrations of 0.1-0.2 g/L resulted in super-large flocs that were separated using a traditional settling approach or a novel screening method. After floc settling or screening, the Fe-fiber-containing sludge was recovered and reused in multiple cycles of treatment, maintaining the process performance with a reduction in coagulant demand of > 60% in multiple cycles, due to the ability of the Fe-grafted fibers to recycle the coagulant. Imaging and chemical characterization of the fiber surface confirms the retention of the alum coagulant on the fibers even after multiple cycles of reuse. An analysis of the supernatant water revealed that the Fe-grafted fibers also improved the removal of natural organic matter (NOM, precursor of disinfection by-products in drinking water) and did not lead to increased soluble iron levels in the water. In addition, the fibers were capable of removing the target contaminant phosphorus from synthetic wastewater with either a reduced coagulant dose or without the presence of coagulant. Hence, we expect that this material could be an easily implementable solution to reduce natural water eutrophication associated with phosphorus in wastewater effluents. Considering their performance, versatility and low-cost, Fe-grafted fibers could be sustainably implemented in water treatment plants globally - for drinking and wastewater applications - to simultaneously deal with soluble and particulate contaminants.