From waste to wealth: Recovery and valorisation of heavy metals in abandoned mine wastewater by layered double hydroxides

Heavy metals have a wide range of applications and as a result their global demand has increased exponentially for over 100 years, leading to their increased extraction from the earth's crust. Successful recycling averts the needs to locate and mine vast amounts of ore and has great potential to ensure the sustainability of heavy metals. Currently, due to insufficient capture many industries directly or indirectly discharge large amounts of metal waste into water streams.
In the UK a major cause of water pollution is that of abandoned metal mines for metals including iron, lead, copper and tin. Over the past 50 - 60 years mining activity in the UK has decreased due to cheaper extraction overseas causing thousands of mines to shut down, resulting in metal contaminated wastewater leeching into local rivers and water systems. A study partaken by the Environment Agency between 2009 - 2012 found that 226 water bodies over England and Wales had been impacted by mine abandonment. It is estimated that it will cost £374 million to remediate water-related environmental problems associated with non-coal mines, with £334 million allocated towards the treatment of mine water.
There are various methods of removing heavy metals from wastewater however adsorption is often regarded as the most effective and economic of these methods due to its simple applicability and low cost when implemented. Layered Double Hydroxides (LDHs) are a layered brucite like materials consisting of positively charged M2+/M3+ sheets balanced by hydrated interlayer regions intercalated with anions. LDHs show great promise as adsorbents for metal contaminated wastewater due to their ease of preparation and tunable binding affinities to specific metals, resulting from the diverse nature of the interlayer anions. Industrial application of LDHs has previously been limited due to the slow co-precipitation synthesis method. Recently however a method of producing nanoparticles continuously has been adapted at the University of Nottingham for LDHs which offers a quick and easy approach to their synthesis on a kilogram/tonne scale.

proposed solution and methodology

This research programme aims to provide industrially applicable LDHs for the effective removal of heavy metals in water bodies impacted by the pollution from abandoned metal mines in the UK. More specifically;

LDHs will be synthesised by continuous hydrothermal synthesis, a quicker and more scalable method than co-precipitation. There are very few papers highlighting synthesis of LDHs by continuous hydrothermal synthesis thus there is scope to reproduce LDHs synthesised by co-precipitation that showed promising adsorption characteristics for heavy metals.

Following synthesis, a thorough investigation of LDHs sorption behaviour including capacity, kinetics and isotherms will be undertaken. In addition to this the factors affecting metal sorption will be investigated such as pH, temperature, adsorbant loading and the effects of co-existing ions in solution. pH and temperature dependence will also be researched as a method for the regeneration of metal loaded LDHs and recovery of adsorbed metals. To ensure applicability, these adsorption studies will be performed in conjunction with tests in environmental water samples which can contain a variety of other ions and contaminants that have the ability to affect adsorbant performance.

Promethean Particles, a University of Nottingham spin-out, have already demonstrated industrial scale-up of the continuous hydrothermal synthesis method. Provided success is achieved with LDH synthesis and adsorption studies, collaboration with Promethean Particles will allow for synthesis to be attempted on a pilot scale rig system capable of producing 1-10 tons of product per year. The effect of the pilot scale rig system on LDH physical characteristics such as size, composition and shape will be studied and subsequent optimization performed.

This CDT will have a positive impact in the following areas:

PEOPLE. The primary focus is people and training. Industry needs new approaches to reach their sustainability targets and this is driving an increasing demand for highly qualified PhD graduates to lead innovation and manage change in the area of chemicals production. CDT based cohort training will provide industry ready scientists with the required technical competencies and drive to ensure that the sector retains its lead position in both innovation and productivity. In partnership with leading chemical producers and users, we will provide world class training to satisfy the changing needs of tomorrow's chemistry-using sector. Through integrated links to our Business School we will maximise impact by delivering dynamic PhD graduates who are business aware.

ECONOMY. Sustainability is the major issue facing the global chemical industry. Not only is there concern for our environment, there is also is a strong economic driver. Shareholders place emphasis on the Dow Jones Sustainability Index that tracks the performances of the sector and engenders competition. As a result, major companies have set ambitious targets to lower their carbon footprints, or even become carbon neutral. GSK CEO Sir Andrew Witty states that "we have a goal to reduce our emissions and energy use by 45% compared with 2006 levels on a per unit sales basis... " Our CDT will help companies meet these challenges by producing the new chemistries, processes and people that are the key to making the step changes needed.

SOCIETY. The diverse range of products manufactured by the chemical-using industries is vital to maintain a high quality of life in the UK. Our CDT will have a direct impact by ensuring a supply of people and new knowledge to secure sustainability for the benefit of all. The role of chemistry is often hidden from the public view and our CDT will provide a platform to show chemical sciences in a positive light, and to demonstrate the importance of engineering and applications across biosciences and food science.
The "green and sustainable" agenda is now firmly fixed in the public consciousness, our CDT will be an exemplar of how scientists and engineers are providing solutions to very challenging scientific and technical problems, in an environmentally benign manner, for the benefit of society. We will seek sustainable solutions to a wide range of problems, whilst working in sustainable and energy efficient facilities. This environment will engender a sustainability ethos unique to the UK. The CNL will not only serve as a base for the CDT but also as a hub for science communication.
Public engagement is a crucial component of CDT activities; we will invite input and discussion from the public via lectures, showcases and exhibition days. The CNL will form a hub for University open days and will serve as a soft interface to give school children and young adults the opportunity to view science from the inside. Through Dr Sam Tang, public awareness scientist, we have significant expertise in delivering outreach across the social spectrum, and she will lead our activities and ensure that the CDT cohorts engage to realise the impact of science on society. Martyn Poliakoff, in his role as Royal Society Foreign Secretary, will ensure that our CDT dovetails with UK science policy.

KNOWLEDGE. In addition to increasing the supply of highly trained people, the results of the PhD research performed in our CDT will have a major impact on knowledge. Our student cohorts will tackle "the big problems" in sustainable chemistry, and via our industrial partners we will ensure this knowledge is applied in industry, and publicised through high level academic outputs. Our knowledge-based activities will drive innovation and economic activity, realising impact through creation of new jobs and securing the future.