Removal of boron and silicon by a modified resin and their competitive adsorption mechanisms

Boron and silicon are essential trace elements for living organisms. However, these are undesirable in excess amounts owing to the toxic effects of boron on plants, animals, and humans, and the silica scale formation by silicon in water treatment processes. Herein, a new diol-type adsorbent (T-resin) was synthesized by grafting tiron (disodium 4,5-dihydroxy-1,3-benzenedisulfonate) onto an ion-exchange resin (grafting amount is 1.2 mmol/g dry) to separate boron and silicon from a solution. The effects of pH, initial concentration, and coexisting anions, particularly, the effect of the coexistence of silicate ion on the adsorption of boron, were investigated. T-resin showed good adsorption properties for both boron and silicon in a wide pH range (pH 2-10). The adsorption of boron and silicon was effectively described by the Langmuir isotherm, and the maximum adsorption capacities of boron and silicon were 21.25 mg/g and 8.36 mg/g, respectively. In a competitive adsorption system, boron and silicon were simultaneously adsorbed on the T-resin, but the adsorption rate of boron was faster than silicon. However, silicon could replace the boron adsorbed on the resin, indicating that the adsorption of silicon was more stable than boron. B-11 and Si-29 solid state NMR data confirmed the different adsorption mechanisms of the two elements. Boron was adsorbed via two types of complexes, a triangular complex of [LB(OH)], as well as 1:1 tetrahedral complex of [LB(OH)(2)] and 1:2 tetrahedral complex of [BL2], whereas silicon was only adsorbed via a 1:3 octahedral complex of [SiL3].