Boron (B) adsorption by soils affects its bioavailable contents and contamination potential. The objectives of this study were: (a) to investigate the effects of pH variation on B adsorption by topsoil and subsoil samples from an Anionic "Rhodic" Acrudox (RA), an Anionic "Xanthic" Acrudox (XA) - both with a positive balance of charge in Bw horizon - and a Rhodic Kandiudalf (RK); (b) to evaluate the ability of the Langmuir model of simulating the experimental results of B adsorption; and (c) to correlate chemical, physical and mineralogical soil attributes with the values of maximum adsorption (Ads max) and affinity coefficient (K L), derived from the isotherms. To quantify the amount of adsorbed B, batch experiments were carried out using a 0.01 mol L-1 NaCl solution as a support electrolyte containing 0.1; 0.2; 0.4; 0.8; 1.2; 1.6; 2.0 and 4.0 µg mL-1 of B. An increase of B adsorption was observed after raising the pH (between 3.5 and 8.0) and the initial concentration of added B. Larger amounts of B were adsorbed in topsoil samples from RK and in Bw positively charged horizons from acric Oxisols. Boron adsorption was represented by type-C (linear) and type-L (exponential) isotherms and well fitted by the Langmuir model. Ads max and K L, estimated by nonlinear regressions, were not correlated with the pH. At a natural pH, the organic matter (OM) and clay contents influenced Ads max most in the topsoil samples. In the subsoil layers, Ads max was negatively correlated with OM contents and positively correlated with gibbsite contents. At a natural pH, the contents of free and amorphous Fe (hydr)oxides were correlated with K L values from subsoil samples. After pH increase, the oxide contents were correlated with Ads max values from the topsoil samples.
boron; adsorption; isotherms; tropical soils; Langmuir
