RETENTION OF PHENOLIC ACIDS BY NONCRYSTALLINE HYDROXY-ALUMINUM AND -IRON COMPOUNDS AND CLAY MINERALS OF SOILS

A study was made on the adsorption of phenolic acids, which are known to be plant growth inhibitors, by kaolinite, illite, vermiculite, noncrystalline hydroxy-Al and -Fe compounds, and four Taiwan soils. The adsorption sequence of phenolic acids by kaolinite, illite, and vermiculite followed the general decreasing order: p-hydroxyben-zoic acid > p-coumaric acid > ferulic acid > syringic acid > vanillic acid. The internal surfaces of vermiculitic minerals were not of great significance in the retention of phenolic acids. This is attributed to steric hindrance and negative repulsion by the interlayers. The removal of the noncrystalline sesquioxidic components from the soils significantly decreased their adsorption of the phenolic acids. The rate and capacity of the noncrystalline hydroxy-Al and -Fe components for the retention of the phenolic acids were by far higher than kaolinite, illite, and vermiculite. The high rate and capacity of these noncrystalline components for phenolic acids are essentially attributable to the great reactivity of their positively charged Al-OH20.5+ and Fe-OH20.5+ functional groups towards the negatively charged carboxyl and phenolic hydroxyl groups.