The effect of hydroxide solutions on dissolution of organic-carbon in some New Zealand soils

Abstract

The disposal of high pH, hydroxide-based, liquid wastes using land treatment systems is becoming increasingly common in New Zealand. For effective disposal of high pH liquid wastes to land, it is necessary to understand the effects of different hydroxide-based solutions on organic carbon dissolution to ensure soil physical properties are not adversely affected. Single-step and multi-step extraction experiments were designed to investigate the effect of high pH solutions on organic carbon dissolution in four New Zealand soils. In the single-step extraction experiments, soil was shaken with varying concentrations of NaOH and KOH (0.003, 0.01, 0.03, 0.1 and 0.3 M) at a 1:5 soil to solution ratio for 18 h. Organic carbon dissolution occurred at very low concentrations and increased linearly with hydroxide concentration, with up to 45% of the total initial organic carbon dissolved when 0.3 M NaOH was used. KOH dissolved slightly less organic carbon than NaOH, indicating that a cation difference occurred. When the anion was changed to chloride, the amount of organic carbon dissolved was very small (<2% of total initial organic carbon) for all concentrations, indicating that the hydroxide anion was most important in causing organic carbon dissolution. Multi-step experiments involved repeatedly shaking soil with fresh hydroxide extracting solutions, and showed that the difference between NaOH and KOH in dissolving organic carbon decreased as the number of extractions increased. The cumulative amount of organic carbon dissolved increased from about 45% after a single-step extraction to about 75% after five extractions. Organic carbon dissolution with different solutions tended to be higher in an allophanic soil, but similar in soils dominated by phyllosilicate clay minerals. The results indicate that factors such as the chemical composition of the liquid waste and soil type need to be considered prior to land disposal to prevent adverse effects on soil physical properties.