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Soil Remediation

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“3.33 million hectares of farmland that are too contaminated to use”

Map soil pollution,
Remediate ~700,000 ha
Use 95% safely by 2030

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Agricultural soil remediation

the process of cleaning and revitalising soil in farming and agricultural areas contaminated by various pollutants.
Ensuring food safety
Protecting the environment
Enhancing crop yields
Meeting agricultural regulations

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Approach

Past Remove all contaminants and restore the land to pristine conditions

Present Land contamination risk management (LCRM)

Future Sustainable remediation

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Sustainable remediation – based on sustainability assessment

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life-cycle impacts

environmental and socioeconomic impacts

remediation alternatives

One may presume that the use of remediation technology to restore contaminated soil and groundwater would be an inherently sustainable action; however, recent holistic life-cycle based research has revealed that cleanup operations can result in significant environmental and socioeconomic impacts. For instance, certain remediation strategies render high levels of greenhouse gas (GHG) emissions, and others may cause unintended harm to vulnerable groups. With sustainable remediation becoming a key topic among remediation researchers, a review article in Nature Sustainability addressing different methods, approaches and solutions to remediate soil and groundwater pollution would not only further stimulate academic debate, but also promote sustainable practice in the remediation field, thus offering important societal impacts.

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Assessment framework with an ‘agricultural’ category

LAND DEGRADATION & DEVELOPMENT. 2018, 29(4), 1005–1018.

Secondary impacts of soil remediation

ENVIRON. SCI.: PROCESSES IMPACTS.

2018, 20, 266.

JOURNAL OF CLEANER PRODUCTION.

2017, 162, 1157-1168.

Sustainability of agricultural land remediation

Social

Economics

Environmental

Agricultural

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Global questionnaire survey of remediation

practitioners in 22 countries and 4 continents

Statistical Analysis

CHEMOSPHERE. 2018, 225, 295-303.

Stakeholder Engagement

Public meetings

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Regulations

Legal context is primary - compliance
Environmental regulations/enforcement (after an incident)
Land use change (fit for an intended purpose)

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Land contamination risk management (LCRM)

Risk assessment

Preliminary risk assessment
Generic quantitative risk assessment
Detailed quantitative risk assessment

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Land contamination risk management (LCRM)

Remediation options that - Mitigate environmental risks
Feasible options to remove source or break pathway to a receptor
Detailed evaluation of options
Select remediation option(s) – cost; whilst optimising the environmental, social and economic value of the work

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Sustainability analysis

Qualitative (simple but comprehensive)
Semi-quantitative (e.g. pairwise comparison (better/worse), MCA)
Quantitative (e.g. LCA, footprinting)

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Remediation

Develop a remediation strategy
Remediate
Verification
Long-term post remediation monitoring and maintenance

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Remediation

Engineered solutions
Bioavailability/stabilization
Bioremediation

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Engineered approaches

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This Photo by Unknown author is licensed under CC BY.

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Chemical Stabilization�

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Soil Amendments for Agricultural Remediation

Adding organic matter - Compost and manure
Lime and gypsum for pH adjustment - bioavailability

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Biochar

Soil remediation
Soil fertility
Carbon sequestration

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Before we look at my research output - I’d like to introduce you biochar.

In recent years, biochar application has been proposed as kind of a green and innovative solution for achieving multiple goals towards sustainable soil use and management, including carbon sequestration, contaminant immobilization, and soil health enhancement, and I’ll talk about those shortly.

But first, what is this material - biochar?

Biochar is made from the burning of various types of biomass in an oxygen‐limited environment – a process know as pyrolysis. For example, on the top right hand side of the slide you will see some biomass, in this case corn stover, and if you pyrolize that you’re left with a residue that is something like charcoal. In terms of chemistry, the hydrogen-to-carbon ratio and the oxygen-to-carbon ratio will both decrease, making it a very stable carbon rich product.

What interesting is that the properties of biochar are tunable by adjusting the pyrolysis conditions. So you can influence the shift in the chemistry – that’s something that we looked at in a study and will discuss that later.

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Higher pyrolysis temperatures = low yield but high pH and fixed carbon

Rice straw may contain Cd, which poses a risk to crops and humans. The exchangeable fraction dropped from 41% to 5.8% at 500 °C and to 2.1% at 700 °C

JOURNAL OF CLEANER PRODUCTION. 2018, 174, 977-987.

CHEMOSPHERE. 2019, 233, 149-156.

Characterized the non-uniform properties of biochar

as a stabilization technology

Now returning to the tuneable properties of biochar produced at different conditions, we characterised biochar yield, pH and fixed carbon content vary as a function of pyrolysis temperate. This information is important for selecting biochars for different uses or situations. For example, the bio-availability of metals is normally high in low pH soils, therefore high temperature pyrolysis that renders high pH biochar may be desirable. On the other hand, it was observed that chemical functional groups – which are useful because they bind with contaminants - are removed at high pyrolysis temperatures. I also considered the fact that biochar itself can contain contaminants if the biomass was original grown on contaminated land. Therefore, we monitored cadmium levels and its behaviour in biochar, finding that cadmium stability increased with pyrolysis temperature.

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SCIENCE OF THE TOTAL ENVIRONMENT.2018, 619, 815–826.

Parameter	Leaching	Uptake	Yield
temperature	.226*	-.251**	-.280*
precipitation	.175	.468***	.199
mixing depth	.002	.383***	.310**
pyrolysis temperature	.265*	.575***	.388**
surface area	.405**	.453***	n/a
pH	.021	-.554***	-.445***
EC	.19	-.608***	-.077
OC	-.019	.444***	.047
CEC	-.262*	.263**	.102
N	-.188*	-.338***	-.189
P	-.187*	-.402***	-.262*
K	-.228*	-.367***	-.250*

*<0.05, **<0.01, ***<0.001

Characterized factors that affect biochar stabilization performance under farming conditions

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Spiked soil

Real soil accelerated aging

Biochar effectiveness is limited in Hg contaminated soils

SCIENCE OF THE TOTAL ENVIRONMENT. 2018, 621, 819–826.

Increased to 99%

Developed a modified biochar technology
Low cost rice husk waste

While the effectiveness of biochar in immobilizing heavy metals and restoring degraded land has been well documented and shown in the field, as we saw on the previous slide, its long‐term effectiveness is still of debate. Moreover, while biochar can stabilize a range of different heavy metal types, its effectiveness can be somewhat inconsistent for certain contaminants, for example, mercury. Therefore, various modification strategies have been explored to strengthen the immobilization capability of biochar. I developed a modified biochar technology specifically for this purpose. I wanted to make this technology low cost, so I used rice husk – an very common agricultural waste in China – which was modified with sulfur. I found that the modification step greatly improved the remediation efficiency, because it forms mercuric sulfide as a stable product. When we applied this technology to a real farm soil and placed it under accelerated aging in the lab, I found that it was able to remain effective for 75 simulated years, which was longer and more effective than un-modified biochar.

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ENVIRONNENT INTERNATIONAL. 2019, 130, 104945.

JOURNAL OF HAZARDOUS MATERIALS. 2019, 121849.

SCIENCE OF THE TOTAL ENVIRONMENT.

2019, 663, 568–579.

Nature-based solution:

low cost, les energy and resilience

Bioremediation

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Harnessing Plant-Microbe Interactions in Soil Cleanup

Bioremediation strategies leverage the interplay between plants, microbes, and heavy metal(loid)s to detoxify soil.
These strategies involve the use of resilient plant and microbial species that can thrive in heavily contaminated agricultural soil.
Some of these organisms adsorb heavy metal(loid)s or release compounds that bind with them, altering their bioavailability and toxicity.
Other species are capable of extracting and removing heavy metal(loid)s from the soil environment.

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Sustainable Soil Cleanup with Bioremediation

Bioremediation stands out as a more sustainable alternative to traditional techniques like soil washing.
Conventional methods may harm living organisms and soil organic matter, posing risks to long-term soil health and productivity.
Bioremediation offers numerous sustainability advantages, including reduced costs, enhanced worker safety, and smaller environmental footprints over its lifecycle.
This shift towards nature-based solutions maximizes economic, social, and environmental benefits in soil remediation.

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Phytoremediation

Phytoremediation utilizes both native and introduced plant species, including genetically modified ones, for soil decontamination.
It can be adapted to different plot sizes by planting an appropriate number of selected phytoremediation plants and considering natural biogeochemical processes linked to plant growth, metal(loid) speciation, and soil changes.
Phytoremediation techniques encompass phytostabilization, where root exudates reduce metal bioavailability in the rhizosphere, and phytovolatilization, which leverages plant evapotranspiration to transfer contaminants from soil to the atmosphere.

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Phytoremediation

'Phytoextraction' is the most commonly used and extensively studied phytoremediation technique. In this method, plant species absorb heavy metal(loid)s from the soil through their roots, with the metals accumulating in the plant's above-ground biomass, which is subsequently harvested.
Typically, the harvested biomass is incinerated, resulting in metal-concentrated bottom ash, which is usually disposed of in landfills.
Proper filtration or scrubbing techniques are needed to address health risks associated with particles produced during biomass combustion.
Harvested biomass can serve as a feedstock for bioenergy production or be pyrolyzed to create biochar, with due safety considerations.

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Plant selection

Plant selection is a crucial phase in phytoremediation, with different species displaying varying capabilities in uptaking or immobilizing specific contaminants.
Indigenous plant species are often favored as they are well-suited to local environmental conditions, but introduced species may be necessary to expedite the remediation process.
Hyperaccumulators, which are plants that can extract significant amounts of heavy metal(loid)s, are particularly advantageous for remediating sites with high levels of heavy metal(loid) contamination.

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Plant selection

However, hyperaccumulators typically have a limited capacity to uptake multiple heavy metal(loid)s, making them less suitable for soils contaminated with diverse metal(loid) species.
In such cases, fast-growing, high-biomass phytoremediation plants like willow, eucalyptus, and poplar trees can be employed to extract a broad range of heavy metal(loid)s from the soil.
Using these plants may temporarily inhibit agricultural production for several years or even decades due to their relatively slow metal-extraction rates.
Farmers can consider implementing intercropping techniques, allowing the simultaneous growth of phytoremediation plants alongside agricultural crops, to mitigate the impact on agricultural productivity.

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Plant selection

Research efforts have been directed towards identifying hyperaccumulator species that can coexist with food crops, enabling soil remediation while safeguarding food crops from contamination.
Another research avenue explores the use of crop plants that can absorb contaminants but do not accumulate them in edible parts. For instance, crops may yield grains suitable for animal consumption while concentrating contaminants in shoots or roots, which can then be extracted using phytoextraction techniques.
One limitation of this approach is that heavy metal(loid) extraction rates can be relatively slow, and the use of such plants may hinder the growth of more economically valuable crops.

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Laboratory Studies on Phytoremediation:

Many laboratory studies have been conducted using artificially spiked soils with heavy metal(loid) concentrations hundreds or thousands of times higher than those found in actual contaminated sites.
The purpose of these studies is to identify hyperaccumulator species more easily and gain a better understanding of the molecular mechanisms involved by subjecting plants to high stress levels.
However, caution is necessary when extrapolating data from such experiments to real-world field operations due to the vast differences in contamination levels.

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Increasing Field Trials for Environmental Relevance:

In recent years, there has been a growing number of field trials aimed at verifying the effectiveness of phytoremediation strategies at more environmentally relevant concentrations.
These field trials assess the influence of various environmental factors on the efficiency of phytoremediation.
Phytoremediation field trials have been conducted globally, with China leading in the number of trials. Large-scale field trials (>500 m2) have been carried out in countries like Switzerland, Germany, and France to evaluate the resilience, stability, suitability, and effectiveness of phytoremediation plants under diverse conditions.

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Increasing Field Trials for Environmental Relevance:

The first large-scale phytoremediation field trials for heavy-metal(loid)-contaminated soils were conducted in the early 1990s, showing promise for reducing metal concentrations on productive land.
Greenhouse pot studies and small-scale outdoor field trials have confirmed the effectiveness of various hyperaccumulator species and identified practices to enhance uptake levels.
Factors such as plant density, initial plant size, cropping and harvesting strategies, and soil heterogeneity have been recognized as crucial factors affecting success in these studies.
Long-term studies reporting annualized treatment efficiencies are preferred over shorter trials, as influencing parameters can vary during field treatments.

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Hectare-Scale Field Trials:

More recently, larger hectare-scale field trials have been conducted, resulting in lower variability in results compared to smaller studies.
An agricultural trial across 11.1 hectares in China, for example, successfully reduced soluble concentrations of lead, cadmium, and arsenic by significant percentages using hyperaccumulator species.
These larger field trials demonstrate the promise of phytoremediation as a cost-effective and efficient remedial approach compared to traditional alternatives.