Silicate sorption and desorption by a Si-deficient soil - Effects of pH and period of contact

The effects of pH (5.0, 6.0 and 7.0) and reaction time (2 h-60 d) on sorption/desorption of Si by a Si-deficient soil were investigated. Sorption isotherms for Si were linear indicating that with increasing surface coverage precipitation reactions were occurring on the sorption surfaces. Sorption of Si was increased as pH was increased from 5.0 to 7.0. Increasing the pH also resulted in a decrease in the percentage of sorbed Si that was subsequently desorbed over 10 desorption cycles. Similarly, increasing the pH during desorption (from 5.0 to 7.0) decreased the percentage Si desorbed as well as the percentage of Si that was released during the first desorption cycle. Thus, increasing pH not only increased the magnitude of sorption but the Si that was sorbed was held more strongly than at lower pH values. Increasing the period of sorption (up to 60 d) resulted in increased Si sorption thus demonstrating the existence of slow reactions between added Si and soil surfaces. Increasing contact time also decreased the percentage sorbed Si that could be desorbed over 12 desorption cycles and the percentage of total Si that was desorbed during the first desorption cycle. As the period of contact during sorption is increased subsequent desorption involves reversal of slow reactions and it is apparent that these are not easily or rapidly reversible. After a 60 d sorption period, only 49% of sorbed Si was desorbed over 12 desorption cycles. It was concluded that over a period of several months sorption can become a process by which Si is sequestered into a non-mobile form so that it cannot be regarded as simply a retention mechanism.