Phosphonate removal from membrane concentrate by electro-coagulation

Abstract

In this study, the efficiency of electrocoagulation (EC) with iron electrodes was applied to remove two phosphonates, 1-hydroxyethylidene-1, 1-diphosphonic acid (HEDP) and nitrilotris(methylene) triphosphonic acid (NTMP) from concentrates. This work provides a detailed description of the experimental procedure and results on phosphonate removal and recovery from different electrolytes, including synthetic and real reverse osmosis (RO) membrane concentrates. This research showed high selectivity of EC, removing 100% and 80% of the NTMP and the HEDP respectively, confirming no competition with sulfates, nitrates, or silica. When experimenting with other electrolytes, calcium showed to be critical in enhancing the flocculation process, while calcium carbonate precipitation contributed to capturing the phosphonates from the concentrate. The produced iron oxide (sludge) was confirmed as goethite and akaganéite, and finally transformed into hematite, indicating the oxidation from Fe2+ to Fe3+ during the EC process. After the iron precipitate collection, an alkaline wash of the sludge was enough to recover 100% of the initial phosphorus from the NTMP phosphonate. However, further research is needed to optimize the recovery procedure and to improve the results with the HEDP. 70 and 140 A·m−2 current densities were optimal to bring HEDP and NTMP concentrations down to 32 μM (1 mg·L−1) in only 30 and 10 min respectively. In these conditions, the operational costs, 1.10 and 0.03 €·m−3 of treated concentrate, were estimated for HEDP and NTMP respectively. Even when EC has been widely studied for phosphate removal, this technique has been barely applied to treat concentrates containing phosphonate-based antiscalants. EC opens new possibilities for phosphonates and phosphorus to be removed and recovered respectively from membrane and other concentrates.