BORON ADSORPTION ON ALLOPHANE WITH NANO-BALL MORPHOLOGY

概要

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Two types of reaction scheme were proposed for boron (B) adsorption at equilibrium B concentrations of less than 2 mM on nano-ball shaped allophane with its fundamental structure of aluminum orthosilicate.(i)Al-OH+B(OH)_3→Al-O-B(OH)_2+H_2O (ii)Al-OH+B(OH)_4^-→Al-(OH)-B(OH)_3+OH^- With the aluminol groups(Al-OH)included in the structure, the reaction(i)occurs below equilibrium pH of about 10, whereas the reaction(ii)does at equilibrium pH of higher than about 8.Calculated Langmuir parameters suggested that the aluminol groups of nano-ball allophane had higher affinity for B(OH)_4^- than B(OH)_3.Because the content of the aluminol groups increase with decreasing Si/Al ratio of allophane, B adsorption maxima were higher for KyP sample(Si/Al=1.34:2)than for KnP sample(Si/Al=1.98:2)irrespective of equilibrium pH.Release of Si from allophane samples during B adsorption on them was much smaller than the case of phosphate (P) adsorption previously studied.This indicates that B has less ability than P for replacing Si in the structure of nanoball allophane and the main B adsorption site is the aluminol group at the pore of the allophane nano-ball.