Organic materials retain high proportion of protons, iron and aluminium from acid sulphate soil drainage water with little subsequent release

Abstract

When previously oxidised acid sulphate soils are leached, they can release large amounts of protons and metals, which threaten the surrounding environment. To minimise the impact of the acidic leachate, protons and metals have to be retained before the drainage water reaches surrounding waterways. One possible amelioration strategy is to pass drainage water through permeable reactive barriers. The suitability of organic materials for such barriers was tested. Eight organic materials including two plant residues, compost and five biochars differing in feedstock and production temperature were finely ground and filled into PVC cores at 3.5 g dry wt/core. Field-collected acidic drainage water (pH 3, Al 22 mg L and Fe 48 mg L) was applied in six leaching events followed by six leaching events with reverse osmosis (RO) water (45 mL/event). Compost and biochars increased the leachate pH by up to 4.5 units and had a high retention capacity for metals. The metal and proton release during subsequent leaching with RO water was very small, cumulatively only 0.05-0.8 % of retained metals and protons. Retention was lower in the two plant residues, particularly wheat straw, which raised leachate pH by 2 units only in the first leaching event with drainage water, but had little effect on leachate pH in the following leaching events. It can be concluded that organic materials and particularly biochars and compost have the potential to be used in acid drainage treatment to remove and retain protons and metals.

Assessment of the Binding of Protons, Al and Fe to Biochar at Different pH Values and Soluble Metal Concentrations

“…The eucalyptus biochar had 4% CaCO 3 ( Table 1), therefore some protons will also be neutralised by carbonates [44,45], although we adjusted to a set pH in the experiments. In summary, the results suggest that biochar has potential for metal and proton removal over a wide range of soluble metal concentrations, as they may occur in drainage channels of acidic ASS, which is in agreement with our previous studies [8,9]. Pilot scale field trials are required to confirm this.…”

“…100). The ANC (3.8% CaCO3), CEC (39 cmol kg −1 ) and surface area (2.5 m 2 g −1 ) of the biochar were moderate compared to other biochars (for details see Dang et al [8]), whilst the extractable Al and Fe concentrations were high. The dominant functional groups were Alkyl, Aryl, and O-Aryl C. The biochar had a high proton binding capacity, whereas its capacity to bind OH − was limited as pH increased rapidly when base was added (Figure 1).…”

“…We showed previously that organic materials, such as plant residues, composts, and biochar retained large amounts of protons and Al and Fe from ASS drainage water and released less than 1% of that retained when subsequently leached with pure water [8,9]. Among the materials that were tested, retention was greater in biochars and composts than in plant residues.…”

“…More systematic studies are needed to better understand proton, Al and Fe binding to biochar to optimise its use in semipermeable barriers for ASS drainage water. The main objective of this study was to evaluate the proton and Al and Fe retention capacity of eucalyptus biochar produced at 550 • C that had showed high Al and Fe retention in our earlier studies [8,9]. Proton sorption to biochar was tested by acid-base titration in the absence of metals.…”