Effect of Dissolved Organic Carbon in Recycled Wastewaters on Boron Adsorption By Soils.

In areas of water scarcity, recycled municipal wastewaters are being used as water resources for non-potable applications.  Such wastewaters often contain elevated levels of dissolved organic carbon (DOC) and solution boron (B).  Boron adsorption was investigated on eight arid-zone soils from California and one from Arizona as a function equilibrium B concentration (0-100 mg L^-1) and solution pH (4-10).  The B equilibrating solutions were prepared using either recycled wastewaters or simulated wastewaters.  The simulated wastewaters were synthesized to have the same chemical composition as the recycled wastewaters but contained no DOC.  Boron adsorption on the soils increased with increasing solution pH, reaching an absorption peak near pH 9, and decreasing with further increases in solution pH.  The pH dependent amounts of B adsorption were not statistically significantly different at the 95% level of confidence for both types of waters.  For all soils, B adsorption as a function of solution B concentration conformed to the Langmuir adsorption isotherm equation.  Boron adsorption maxima, obtained using the Langmuir isotherm, were not statistically significantly different at the 95% confidence level for both types of waters.  The constant capacitance surface complexation model was able to predict B adsorption as a function of solution pH using the soil chemical properties:  aluminum oxide, inorganic carbon, organic carbon contents, and surface area.  The magnitude of B adsorption as a function of solution pH and equilibrium solution B concentration was unaffected by the presence of DOC in the recycled wastewaters.  This result indicates that the treated recycled wastewaters, due to their very low DOC contents, will not affect B adsorption behavior of soils when used for landscape irrigation.