Exchange between the SO
42− ions in ettringite and borate is believed to occur during borate removal from wastewater by the Ca(OH)
2–Al
2(SO
4)
3 addition method. To investigate borate uptake by ettringite, we synthesized ettringite in the presence of H
3BO
3 and prepared suspensions of ettringite in H
3BO
3 solution. At the theoretical Ca(OH)
2 / Al
2(SO
4)
3 ratio for ettringite, with an increase in the H
3BO
3 / Al
2(SO
4)
3 ratio, the ettringite concentration decreased but the concentration of amorphous compounds increased; further, the amount of borate uptake by the precipitate formed also increased. Upon alkalization of the H
3BO
3–Al
2(SO
4)
3 solution to pH = 10 with NaOH solution, borate-containing amorphous Al(OH)
3 was precipitated. It was proposed that the borate coprecipitated along with amorphous Al(OH)
3 (which showed boron uptake) and did not undergo exchange with SO
42− ions in ettringite; this was strongly supported by the results of IR and TG analyses performed on the boron-containing precipitates. At a high H
3BO
3 / Al
2(SO
4)
3 ratio, ettringite decomposed to afford amorphous Al(OH)
3, and borate coprecipitation was promoted. Upon suspension in H
3BO
3 solution, the ettringite precipitate showed low boron uptake, which was presumably due to the substitution of SO
42− in ettringite with B(OH)
4−.
View full abstract