Effects of solutions with high monovalent cation concentrations and high pH on soil properties

Abstract

The effects of solutions with high monovalent cation concentrations and high pH on selected soil physical and chemical properties were investigated using a range of New Zealand soil types.

Some industries produce liquid wastes which are characterized by high pH and high monovalent cation concentrations. However, land treatment of these liquid wastes have been observed to cause adverse effects on soil properties, especially structural deterioration of the 0-10 cm depth of soil. To establish the mechanisms which cause the observed adverse effects, laboratory experiments were conducted to investigate the processes which occur when high pH solutions which contain monovalent cations (NaOH and KOH) are applied to soil. To isolate pH and cation effects, NaCl and KCl solutions were also used as treatments throughout this study. In addition to laboratory experiments, a field trial was conducted to examine the short term (12 months) effects of a surface applied amendment (gypsum, applied at 5 t ha⁻¹) on selected properties of a soil which had been irrigated with high pH dairy factory liquid wastes.

Single-step extraction experiments showed that hydroxide solutions dissolved organic carbon (OC) through the range of concentrations typically found in high pH liquid wastes (i.e. pH 10.5 - 13.5) and OC dissolution increased with increasing hydroxide concentration (i.e. increasing solution pH). Although NaOH dissolved more OC than KOH in the short term (after a single extraction), repeated treatment of soil with fresh hydroxide solution (using multi-step extractions) caused OC dissolution to increase though no cationic differences were measured in the longer term. In contrast to hydroxide solutions, chloride solutions dissolved minimal amounts of OC.

The saturated hydraulic conductivity (Kₛ) of the soils used in this study decreased over time when hydroxide solutions were used as influent solutions. Aggregate stability was also shown to decrease when soils were treated with hydroxide solutions. A two-stage process was proposed to explain the decrease in Kₛ when high pH solutions were applied to soil. First, OC is dissolved at the surface of the soil (i.e. 0-1 cm depth of soil) where the high pH solution is in contact with soil aggregates. The time required for dissolution to weaken aggregates being concentration dependent. Second, once a critical amount of OC had been removed, increased repulsion of soil particles (associated with increased negative charge on variable charge components in the soil due to increased pH) was thought to cause dispersion and deflocculation of clays and movement of the dislodged particles into pore spaces resulted in decreased Kₛ. In contrast to hydroxide solutions, aggregate stability was unaffected when soil was treated with chloride solutions and Kₛ was maintained or increased when chloride solutions were used as influent solutions. Decreasing the electrolyte concentration of the soil solution (by replacing the influent chloride solutions with distilled water) caused Kₛ to decrease, consistent with widely accepted diffuse double layer theory.

Cation exchange capacity (CEC) increased with increasing hydroxide concentration whereas chloride solutions had no effect on CEC. The increase in CEC in hydroxide-treated soil was attributed to negative sites being generated on components possessing variable charge characteristics. In general, higher exchangeable sodium percentage (ESP) and exchangeable potassium percentage (EPP) were found in hydroxide-treated soil compared to chloride-treated soil and were attributed to the new negative sites being counter-balanced by the monovalent cation present in the hydroxide solution (either Na⁺ or K⁺). Equations commonly used to predict ESP and EPP values from the composition of the soil solution (Richards, 1954) accurately predicted ESP and EPP in chloride-treated soil but did not accurately predict ESP or EPP in hydroxide-treated soil. These prediction equations do not take into account the increased total negative charge of the soil system and can therefore not be used to accurately predict ESP and EPP when high pH solutions are applied to soil.

The field trial showed that gypsum application increased infiltration rates in the irrigated soil by decreasing both the ESP and EPP of the soil and increasing the exchangeable Ca²⁺ concentration. Generally, gypsum did not affect any of the other measured properties (pH, OC, CEC, and bulk density) over the duration of the trial. In non-irrigated soil, the lack of effect of gypsum on any of measured soil properties was attributed to the high initial exchangeable Ca²⁺ concentration and low exchangeable Na⁺ and K⁺ concentrations compared to the irrigated soil.

It was concluded that for successful land treatment of liquid wastes with high monovalent cation concentrations and high pH, pH neutralisation of the liquid is essential to decrease the likelihood of OC dissolution and a build-up of negative charge on soil particles occurring. In addition, divalent cations should either be added to the liquid waste prior to disposal or should be applied to the soil in a relatively soluble form, in order to decrease the magnitude of problems associated with accumulation of monovalent cations on exchange sites. A conceptual model was proposed to describe the reactions that occur in soils when liquid wastes with high monovalent cation concentrations and high pH are applied using land treatment systems.