Importance of surface charge characteristics when selecting soils for wastewater re-use

Abstract

Land application represents a viable option in the management of wastewater such as landfill leachate. The surface charge and ion adsorption characteristics of the receiving soil will play an important role in the success of this disposal strategy. The surface charge characteristics of 6 soils with contrasting chemical and physical properties were studied using charge balance principles. The point of zero net proton charge density, point of zero salt effect and point of zero net charge, and the concentrations of permanent and variable charge as a function of pH, were determined for each soil. The presence and magnitude of permanent negative surface charge was largely dependant on clay mineralogy. All soils displayed increasing negative surface charge density with increasing pH. The magnitude of increase for any particular soil was dependent on the type and amount of colloids exhibiting variable surface charge. The increased negative surface charge was attributed to the ionisation of functional groups on organic matter, hydrous Fe and Al oxides, and edge sites of kaolinite and smectite. The variation in surface charge with pH conformed to charge balance principles for soils containing a mixture of both permanent and variable surface charge, but not for soils containing predominantly permanent surface charge. Proton-consuming processes (e.g. adsorption on permanent charge sites, neutralisation of inherent alkalinity) influenced the overall charge balance, particularly for soils with predominantly permanent surface charge. Ammonium adsorption from landfill leachate was measured as a function of pH for each soil type. Increased negative surface charge resulted in increased ammonium adsorption, and for most soils the preference of the newly created adsorption sites for ammonium remained relatively constant. Since many soils contain a mixture of colloids with permanent and variable surface charge, these charge characteristics and their effect on individual soil properties, contaminant availability, and contaminant mobility must be taken into account when evaluating the suitability of land for wastewater re-use.