Soil provides multiple important functions such as provision of food and raw materials, a platform for urban development and human wellbeing, and a filtering and transforming medium for water, nutrients and carbon. Functioning soils are necessary for ecosystem service delivery, climate change abatement, food and fibre production, and freshwater storage. 

Yet, key policy instruments and initiatives for sustainable development have not fully recognised how contaminated soils compromise the addressing of major challenges such as food and water security, biodiversity loss, climate change and energy sustainability. The presence of contaminated soils causes direct acute and chronic health risks, and limits effective land use for food production, living space and economic development.

It is estimated that across the European Union, 350,000 sites are affected by soil contamination. The expectation is that this will continue to grow with nearly 3 million polluting activities and estimates of the cost of treatment reaching €4.8 billion (EEA, 2015). In the UK alone, there are nearly 300,000 potentially contaminated sites with an economic value over £1 billion. Heavy metals, together with hydrocarbons, are the most frequent contaminants found at contaminated sites. Similarly, in other countries like China, the fast rate of urbanisation along with huge expansion of manufacturing industry has led to the emergence of significant soil and water contamination problems. In 2014, a national soil survey performed by the Chinese government concluded that up to 16% of all soils, and nearly 20% of all farmlands, were contaminated by organic and inorganic chemical pollutants. In India, recent economic development combined with poor environmental and waste management has also led to rapid land-use change and the emergence of significant soil and water contamination and brownfield problems across the country.

Over the past 20 years there has been an increasing drive towards more sustainable treatment-based solutions for contaminated land management as opposed to removal or containment actions, or ‘intensive’ treatments with high requirements for onsite infrastructure, energy and resource use. This has led to the emergence of very successful in situ bioremediation treatments. However, the complexities of both soils and the chemical hazardous mixtures encountered tend to affect the application spectrum and efficiency of bioremediation. Further to this, most of the current bioremediation systems and approaches for treating contaminated soil do not satisfy the end-user needs in terms of efficiency, sustainability and cost-effectiveness. Such technologies rely on physical, chemical or bio-based methods, the former two of which are generally prohibitively expensive, whereas all are either non-site-specific, do not work, or are quite slow. Thus, the requirement to make contaminated land fit for use places a large economic burden on stakeholders, and there is a pressing need for knowledge, products and new technologies that can remediate such land rapidly, cost-effectively and sustainably. In this respect, pyrolysis of a range of waste feedstock sources to produce biochar to apply to contaminated and degraded soils could be one way in which to tackle these problems. It further holds huge potential to be developed into an effective agent to treat contaminated land.

Biochar has been extensively studied as a soil amendment to enhance soil quality, crop production and to enhance the bioremediation activities of autochthonous populations of soil microbes for treating contaminated soils. However, research on biochar has not progressed much further than observing what happens within the soil, the rhizosphere and to crops during and after its application. Studies testing pre-inoculated biochar with specific microbes/consortia to treat polluted soils have shown promising results, but such studies are very few, and any consideration to match specific microbes with the properties of biochar is still in its infancy.

Biochar is the carbon-rich product of the thermochemical conversion of biomass, such as wood, manure, sewage sludge or organic wastes, in an oxygen-depleted environment. On average, 50 metric tonnes of biomass feedstock suitable for biochar production is available in the UK each year, which includes crop, wood and forestry residues, and animal and biodegradable municipal waste. Additionally, there is a significant volume of sewage sludge produced by wastewater treatment plants estimated at 10 Metric tonnes (dry weight) in the EU annually. Biochar from the pyrolysis of such waste materials for use in the remediation of contaminated soil can offer other major beneficial impacts, such as: (i) a climate change mitigation technology because it acts as a carbon sink; (ii) a crop yield enhancer, such as a nutrient supply/growth medium, applied directly to the plants or indirectly to rhizospheric microbes; (iii) it can provide co-benefits in saving water; and (iv) it substitutes for fossil fuels used in the production of other soil-improving agents.

The current situation of contaminated soil might appear bleak, but it does present us with an exciting opportunity for biochar as a combined resource-recovery and remediation strategy, which can drastically reduce future remediation costs and reclaim valuable land, while at the same time unlocking billions of tonnes of valuable resources contained within these waste streams, improving the local environments and welfare and therefore contribute to progress on several sustainable development goals.

Bioremediation via active microbes pre-inoculated onto biochar can be viewed as a complete delivery package to soil as it contains the cellular machinery (i.e. the microbes) that degrade/detoxify the pollutants, and nutrients (provisioned by the biochar) needed to support the microbes. Biochar also provides a surface carrier-support allowing the direct adsorption of the pollutant molecules into its inner pores, which is believed to favour electron transfer with the microbial cells and thus acting as an electron shuttle that accelerates pollutant degradation/detoxification. Pre-inoculated biochar can thus be regarded as more favourable for remediation compared with amendment of soil with non-inoculated biochar as the latter relies on the presence of bioremediation-active microbes in the soil and their interaction with the biochar. However, the variability in the physicochemical and functional properties of different biochars can make matchmaking them with the microbial degraders unreliable and their subsequent use in soil remediation unpredictable. Therefore, future progress in biochar development is expected to centre around ‘tuning’ the properties for tailored bioremediation applications.

Further study is also needed to critically examine the role of microbes in soil remediation and mineralisation processes. Although the combination of functional microbes and biochar amendment has been shown as a promising technology for the green and sustainable remediation of contaminated soils, there is currently a disconnect between our understanding of the contaminant removal processes at the laboratory scale versus the field scale. Therefore future work should seek to bridge this gap by incorporating current knowledge into design considerations and monitoring system performance over multiple years. Moreover, the lifetime of the biochar-augmented medium is another remaining knowledge gap. While media composition can be manipulated to maximise sorption capacity, our understanding of the effects of weathering under actual environmental conditions on long-term performance of degradation is particularly lacking. Therefore, field studies that monitor long-term performance are warranted. This will require interdisciplinary efforts among researchers in chemistry, microbiology, engineering and technology as well as collaborations between researchers and practitioners to ensure effective implementation of this technology into broader practice.