Novel process to remediate land and water contaminated by Acid Mine Drainage, with a focus on South Africa

Lead Research Organisation: Imperial College London
Department Name: Department of Chemical Engineering

Abstract

Acid Mine Drainage (AMD) refers to the acidic discharge generated at active or disused mining sites. The low pH of this discharge leads to the mobilisation of metals, often containing those classed as heavy metals such as Fe, Cd, or Mn. The acidity and elevated heavy metal content of AMD make it environmentally damaging, with the latter generally being considered more important.
AMD is generated by the oxidation of ferrous sulphide minerals, most commonly pyrite (FeS2), which is often enhanced by bacterial activity. While this process can occur naturally, human mining activity increases its rate by increasing the exposure of such minerals to air and water.
Conventional AMD remediation methods are typically chemical. The most common involves the addition of alkaline materials such as lime or limestone to increase the pH of AMD waters.This has the benefit of stimulating the oxidation of some metals, such as iron(II) and manganese(II), and the precipitation of dissolved metals under the form of hydroxides. Other conventional treatments include Anoxic Limestone Drains, membranes, and adsorption. The principal drawback with conventional methods is cost: clean-up of all AMD in the USA using such technologies has been estimated at $400bn. Furthermore, the generation of sludge presents a disposal issue.
Wetlands offer a cheaper alternative to AMD remediation. While exact definitions of wetlands vary, they may generally be described as areas where the soil is usually saturated with water or submerged. The complex interactions within wetlands derive from this confluence between terrestrial and aquatic ecosystems, and this allows them to play an important role in remediation of AMD. The main action of wetlands is reduction of heavy metal concentrations, which occurs through a number of mechanisms. These may be broadly classified as dilution, sorption to soil and sediment, precipitation and co-precipitation due to oxidation/reduction, and phytoremediation. This project will be concerned about phytoremediation, and in particular phytoextraction: the uptake of heavy metals by plants and the subsequent storage of these metals in their above-ground structure. If this is combined with harvesting of the contaminated shoots and leaves, the heavy metals can effectively be removed from the system.
The main aquatic plants species used in wetland phytoremediation, such as reeds and cattails9, are made up of lignocellulosic biomass. This makes them compatible with the ionoSolv process currently being developed by the Hallett Group. The ionoSolv process involves the pre-treatment of lignocellulosic biomass with ionic liquids (ILs), in order to fractionate the biomass into its three major components (cellulose, hemi-cellulose, and lignin). Specifically, the ionic liquids (PILs) dissolve the lignin and hemicellulose, leaving a cellulose-rich pulp. Separation and valorisation of these three natural polymers offers the possibility of a viable biorefinery process, especially following the development of effective and cheap protic Ionic Liquids (PILs). In particular, the delignification of the cellulose pulp often allows higher glucose yields to be achieved by enzymatic hydrolysis, from which bioethanol may be produced by fermentation. The process has been successfully applied to grasses, softwoods, and the particularly recalcitrant hardwoods. Furthermore, the process has been shown to work on heavy metal-contaminated biomass, with the ability to recover the metals from the IL remaining at the end of the process. This would be particularly applicable to heavily contaminated biomass grown on AMD, and would provide an extra product stream of significant value.

Publications

10 25 50

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 31/03/2022
1966514 Studentship EP/N509486/1 01/10/2017 31/03/2021 Anton Edward Firth
 
Description In the first chapter of my PhD, I have performed high-level economic modelling of wetland remediation, comparing it to conventional remediation of wastewater. This showed the range of conditions under which wetland remediation is most economically favourable (typically small to medium-scale water flows and contamination levels), which will hopefully incentivise further adoption of this technology.

In the second chapter, I have studied pretreatment of biomass grown on wetlands using low-cost ionic liquids, for production of high-purity cellulose which may then be further refined for materials applications or further processed to produce biofuels. Multiple promising operating conditions were identified. It was also found that this process is fairly feedstock-independent, allowing a range of different species to be used, and thus not being geographically limited. With a view to scaling up the process, ionic liquid recovery and reuse for pretreatment was carried out, with insight gained into its drawbacks and potential solutions identified and tested.

In the third chapter, I investigated the effect of ionic liquid structure on its acidity, and the subsequent impact on the pretreatment efficacy. A significant and unexpected trend was found, showing that the alkylammonium cation structure can significantly increase the solution acidity. Additionally a new promising ionic liquid was identified, offering similar performance to current gold-standard ionic liquids but at reduced durations.

In the fourth chapter, ionic liquid pretreatment is being carried out on starch-based biomass. Due to the different biomass structure, different ionic liquids must be used, and the process must be further refined. This is still in the early stages. However, as part of this chapter a method has been developed for easier characterisation of the ionic liquid acid-base ratio. This method is to be validated in the near future.
Exploitation Route Many of the experimental processes developed in the project still have scope for further scale-up and optimisation in order to enhance their industrial viability. It is hoped that this will be the groundwork for more rigorous adoption of constructed wetland remediation on a large-scale, by showing that processing of the by-products (i.e. the biomass) can have a significant positive impact on process economics.
Sectors Agriculture, Food and Drink,Energy,Environment,Government, Democracy and Justice