New insight into the changes in metal-phosphonate complexes from the addition of CaCO3 to enhance ferric flocculation for efficient phosphonate removal

Due to the stable chelating effect of organic phosphonates in wastewater, phosphonates with increasing emission are difficult to be removed effectively by traditional ferric salt flocculation, which has posed tough challenges for reducing total phosphorus pollution in recent years. In this work, calcium carbonate (CaCO3) was introduced to work together with the widely investigated flocculant of ferric chloride (FeCl3) to realize an efficient removal of nitrilotrismethylenephosphonic acid (NTMP) at much lower dosage of FeCl3. With an aid of synergy effect from together use of CaCO3 and FeCl3, the remaining concentration as low as 0.16 mg-P/L, far below the sewage discharge limit (0.5 mg-P/L), was simply obtained with a significantly reduced Fe/P molar ratio at only 4, resulting from calcium source donor to form more stable Fe-Ca-P tridentate bridging complexes, high affinity towards ferric ions on CaCO3 surface and slow-release alkaline from CaCO3. A comparison among sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2) and CaCO3 as additives, was carried out to highlight the advantages of using CaCO3 and clarify the mechanism for the greatly improved performance by a set of characterizations including XRD, FTIR, Zeta potential, XPS, SEM-EDS and TG analyses. The addition of CaCO3 in ferric flocculation resulted in further obvious advantages such as 75% shortened settling time and only one-third of sludge volume of the precipitant, beneficial to the sample handling in engineering application. The proposed new approach has been further confirmed to work efficiently on real phosphonate-containing wastewater. Discussion on the interaction between CaCO3 and ferric salts in phosphonate solutions shed new insights into the working mechanism of using CaCO3 for the treatment of phosphonates-containing wastewater.