Plant-based biochar is essentially carbonized biomass resulting from various regimes of pyrolysis. Different biochars exhibit various physical (e.g. porosity and surface area) and chemical properties (e.g. surface alkalinity, oxygen-containing functional groups), and will thus perform chemical functions, for example heavy metal adsorption. Although there has been much published evidence supporting the benefits of biochars in soils, there has been little research on the use of biochars in remediating mine wastes (such as base metal mine tailings) for improved outcomes of phytostabilization. As a result, the aim of the research presented herein was to characterise physical and chemical properties of hardwood (jarrah) and softwood (pine) biochars and their adsorption behaviour under a range of simulated pH and salinity (sulphate) conditions typical in the pore water of base metal mine tailings. The biochars used in the present study were produced by pyrolysis of jarrah and pine wood at approximately 700 °C with a retention time of 30 minutes in a commercial pyrolysis kiln. An activated carbon was chosen as a comparison in all experiments.
Jarrah and pine biochars and activated carbon all exhibited different physico-chemical properties, including surface area, porosity, alkalinity, pH, negative charge density, base cations and the presence of oxygen-containing functional groups, which, in combination, have close bearings on their adsorption capacity for heavy metals. The activated carbon had the highest microporosity and total surface area, followed by jarrah biochar and pine biochar. Pine biochar was only slightly basic (pH 7.8), while the pH of jarrah biochar and activated carbon was 9.5 and 9.9, respectively. This pH difference may be related to their alkalinity, which was generated by the dissolution and exchange of base cations from the carbon surface into the solution. Zeta potential analysis showed that both pine and jarrah biochars carried negative charges on their surfaces under pH 2.0-6.0. Comparatively, under the same pH conditions, jarrah biochar had a higher negative charge density than pine biochar. Based on the NMR analysis, the biochars and activated carbon did not contain significant amounts of oxygen-containing functional groups (e.g. carboxyls and phenolic groups).
The comparative Cu and Zn adsorption capacity of the chars was consistent with their physico-chemical properties, following the order of activated carbon > jarrah biochar > pine biochar. The microporosity of jarrah biochar and activated carbon was one of the most significant factors contributing towards the higher adsorption capacity. The chemical mechanisms responsible for Cu and Zn adsorption may involve: (1) the strong electrostatic attraction between negative charges on carbon surfaces and positively charged Cu and Zn ions in aqueous solution; and (2) pH-induced speciation of free Cu and Zn into hydroxides, due to the alkalinity of the chars. There was a positive relationship between Cu or Zn adsorption and the release of Ca2+, Mg2+, K+ and Na+ into the solution in both the biochars and activated carbon. The negative charge density of the chars linearly increased with increasing pH in the solution. This is consistent with the present finding that Cu and Zn adsorption in the chars increased rapidly and positively with increasing initial pH from 2.0 to 5.0. The effect of solution pH on Cu and Zn adsorption by the chars may also be partially attributed to the pH-induced speciation of free cations into hydroxides, which would have much higher affinity for the negatively charged carbon surfaces. The threshold pH for free Cu speciation into hydroxides in aqueous solution was lower than Zn, which may partially explain the greater increase of Cu adsorption when pH increased compared to Zn adsorption. The effect of increasing salinity from 0 to 0.3 M Na2SO4 on the Cu and Zn adsorption was bimodal: a positive effect was initially observed up to 0.1 M, followed by a negative effect from 0.1 to 0.3 M. Increasing ionic strength of the electrolyte (in this case, Na2SO4) can increase screening of the surface charges (either positive or negative) and decrease either repulsive (between the same charge) or attractive (between opposite charges) electrostatic interactions. These may be caused by the initially enhanced cation (e.g. Ca2+, Mg2+) exchange with increasing Na+ in the solution. As the ionic strength increased to > 0.1 M, a negative screening effect of Na2SO4 may have occurred.
Both biochars may be used in remediating base metal mine tailings in which acidic pH and high sulphate concentrations in pore water may be present. However a much higher application rate (particularly, the softwood biochar) may be required to generate maximum adsorption capacity when comparing the adsorption capacity of activated carbon. Further studies are required to investigate the interactions of these biochars with the solid phase of tailing minerals.