Capacity of organic materials to retain metals and protons released from sulfuric acid sulfate soils

Abstract

Acid sulfate soils (ASS) contain the iron sulfide mineral rich in pyrite which is formed under waterlogged or subaqueous conditions. Upon drainage or disturbance, pyrite can be oxidised to produce sulfuric acid which results in soil acidification and metal release. When rewetted due to rainfall or irrigation, oxidised ASS release large amounts of acidity and soluble metals, predominantly Al and Fe, in ground and drainage water that pose a serious risk to ecosystems, agricultural productivity, human health and other assets. Organic materials have been considered as a low-cost and friendly environment absorbent to reduce acidity and metal concentration in leachate from mine tailings and waste water. However, little is known about the potential of these materials to reduce leaching of protons and metals from sulfuric ASS drainage water and how retention is influenced by properties of the organic materials and form of amendment. Eight organic materials (two plant residues, compost and five biochars produced from a range of food stock sources and varied in production temperature) were used. The aims of this thesis were i) to study the effect of different organic materials on leaching protons and metals from sulfuric soil, ii) to assess the ability of different organic materials to remove protons and metals from ASS drainage water, and iii) to determine maximum capacity to retain proton and metals of a biochar. In the first experiment, sandy sulfuric soil (pH 3.5), collected from Gilman in the Barker Inlet, South Australia, was used to study i) the effect of organic materials on leaching of protons and metals from the soil and ii) how is this influenced by properties of organic materials and amendment forms. The organic materials were either mixed into the soil or placed as a layer under the soil, at a rate of 15 g C kg-1. Then, the soil columns (30 g soil) were leached four times with reverse osmosis (RO) water. In the unamended soil, 60-90% of total protons, Fe and Al were released in the first leaching event with only small amounts being released in the three subsequent leachings. Addition of organic materials to the soil increased the pH of the leachate from 0.2 to 2.2 units, and reduced proton and metal leaching by 50-90%. Cumulative retention of protons, Fe and Al was highest in soil amended with eucalypt biochar and wheat biochar produced at 550 ˚C and 450 ˚C, respectively, but low in wheat straw and compost. Retention of Fe and Al was generally greater when mixing organic materials into the soil than when placed as a layer underneath the soil, but there was a little difference between amendment forms in proton retention. Proton retention was positively correlated with C concentration of the material, while Al and Fe retention was positively correlated with percentage of Aryl and O-Aryl groups and negatively correlated with percentage of O-Alkyl and Di-O-Alkyl groups. Synthetic acid drainage water (pH 3, Al 2 mg L-1 and Fe 28 mg L-1) based on the long term average of drainage water in an area dominated by ASS was used to investigate proton and metal retention by organic materials. In this experiment, drainage water was passed through cores which were filled with organic materials at a rate of 1.5 g C per core over four leaching events (45 ml/event). Biochar and compost increased the leachate pH by up to 4 units. Eucalypt and wheat biochar produced at 550 ˚C and 450 ˚C, respectively, had high retention capacity for protons, Al and Fe. The correlation between retention of protons, Al and Fe with properties of organic materials was similar as in the previous experiment. Retention was lower in organic materials with high release of native Al and Fe (compost, wheat straw) than those with low release. Metal and proton concentration in ASS drainage water can vary substantially. The aim of the third experiment was to study retention capacity at high metal concentrations and assess their subsequent release by uncontaminated water. Drainage water was collected in the field in autumn (pH 3, Al 22 mg L-1 and Fe 48 mg L-1). Cores with organic materials at 3.5 g dry wt/core were leached six times with drainage water followed by six leaching events with RO water. When leached with drainage water, biochar and compost increased the leachate pH by up to 4.5 units and retained almost 100% of added protons. Biochars retained cumulatively over 90 % of added Al and Fe, whereas 50-80% of added Al and Fe was retained in wheat and pea straw. Less than 1% of retained protons and metals were released with subsequent leaching with RO water. It is well-known that pH plays an important role in metal speciation, solubility and complexation. A batch experiment was conducted to assess the retention capacity of eucalypt biochar produced at 550 ˚C. The biochar was added at 1% (w/v) to solutions with varying concentrations of protons, Al and Fe and shaken for 24 h. In the absence of metals, the biochar had high proton retention, up to 0.035 mmol of acid was adsorbed in the material. The batch experiment with metals was carried out at pH 4 and pH 7 with Fe or Al at 10-6, 10-5, 10-4, 10-3, and 10-2 M. It showed that the biochar had a high retention capacity for Al and Fe, at high concentrations over 80% of soluble metals was retained. In another experiment, both Al and Fe were added at different ratios, increasing concentrations of one metal did not reduce retention of the other. It can be concluded that addition of eucalypt biochar and wheat biochar produced at 550 ˚C and 450 ˚C, respectively, can strongly reduce leaching of protons and metals from sulfuric soil and drainage water of acid sulfate soils. The retention of protons and metals to organic materials was strongly correlated with properties of organic materials.