Enhanced Cr(VI) removal based on reduction-coagulation-precipitation by NaBH4 combined with fly ash leachate as a catalyst

- Self decomposition of NaBH4 can provide alkalinity for Cr(III) precipitation.
- Improvement from fly ash leachate reflected in Cr(VI) reduction and settling effect.
- The improving reduction efficiency of NaBH4 owed to its enhanced hydrolysis.
- H+ produced by Al and Fe hydrolysis was the driving force for Cr(VI) reduction.
- NaBH4 can make Fe3+ in FA leachate form nano iron at initial pH 2.8 to 3.0.

A novel treatment technology for Cr(VI) removal from aqueous solution using NaBH4 as the reducing agent combined with fly ash leachate was introduced in this study. The effects of the NaBH4 dosage, initial pH, final pH and amount of fly ash leachate on Cr(VI) removal were investigated. The results proved that the reduction capability of NaBH4 for Cr(VI) could be improved by adding fly ash leachate. The removal of 100 mg/L Cr(T) in the absence and presence of fly ash leachate with treatment by NaBH4 increased from 65.93% to 99.81% at an initial pH of 3.0. H+ produced by the hydrolysis of Fe3+ and Al3+ in fly ash leachate provided a buffered acidic condition that catalyzed the hydrolysis of NaBH4 and also slowed the increase in pH caused by surplus BH4−, which is beneficial for improving the reduction efficiency of Cr(VI). Additionally, the Fe-Al-Si complex from the fly ash leachate accelerated the settling velocity of flocculation particles by adsorption and co-precipitation. It was also believed that large quantities of Cr(III), Ca2+, Mg2+ and Al3+ led to a better settling effect by co-precipitating with Cr(OH)3 under alkaline conditions. The Fe-Al-Si complex also enhanced the adsorption of Cr(VI) and the immobilization of Cr(III) generated by reduction and thus promoted Cr(T) removal. Atomic force microscopy (AFM) imagery indicated that NaBH4 caused Fe to form nanoscale zero valent iron (nZVI) over an initial pH range of 2.8-3.0, which may also be a reason enhanced Cr removal. In a way, the formation of nZVI may have a negative influence on the hydrolysis of Fe3+.