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ReSoil®, our patented soil remediation technology is the single commercially available option that efficiently removes Pb and other toxic metals and metalloids from contaminated soils and sediments, recycles reagent and process waters, generates no liquid wastes, produces no toxic emissions from remediated soil and preserves remediated soil as a natural resource.

 

Soil contamination with Pb and other toxic metals is ubiquitous 


Soils are an omnipresent factor of human existence and valuable, limited resource. They are often contaminated with a number of toxic metals which doesn’t biodegrade or decay, with Pb presenting the most pervasive and persistent risk to human health¹. Urban soils act as an integrator of decades of pollution by Pb-based paint and emissions from the combustion of leaded gasoline. In the USA, exterior Pb paint was used in 89% of residential structures before 1978.² As an example of urban contamination, out of 49 analysed playgrounds in Vilnius, 21 had moderately hazardous to hazardous levels of Pb contamination.³ Both racial/ethnic and income disparities have been documented with respect to exposure to Pb in soils.⁴

 

Shooting guns at firing ranges is an occupational necessity for military, police officers and security personnel. In the USA alone an estimated 16.000-18.000 firing ranges exist. Following the shooting activity the soil at firing ranges accumulate substantial quantities of Pb.


One of the most important sources of soil Pb is ore mining/smelting: according to a global inventory (late 1980s), about 356-857×10⁶ kg Pb was released annually.⁵ Same of the major miming/smelting locations in Europe are Noyelles-Godault/Auby, Mortagne-du-Nord and Bazoches-les-Gallérandes in France, Pribram in Czech Republic, Olkusz and Bukowno in Poland, Avonmouth in UK, Harjavalta in Finland, Rönnskär in Sweden, Lommel in Belgium, Kuklen in Bulgaria, Kosovska Mitrovica in Kosovo, Veles in Macedonia.⁶ Emissions decreased in western countries over the last 50 years due to the installation of efficient flue gas cleaning systems.⁷ However, the increasing industrial activities on other continents could significantly affect global emissions. For example, up to 2007 cumulative emissions from mining, ore dressing, and smelting in China were about 1.62 ×109 kg Pb.⁸


Pb is highly toxic to humans. Any exposure is considered to be potentially harmful, no threshold for adverse effects has been identified.⁹ In particular children are a highly vulnerable group due to accidental mouth ingestion.¹⁰ Adults take up 10–15% of ingested Pb, whereas children may absorb up to 50% via the gastrointestinal tract.¹¹ Ingestion of soil accounts for more than 80% of the daily Pb exposure.¹² The high gastrointestinal uptake and the permeable blood­ brain barrier make children susceptible to Pb exposure and subsequent brain damage, neurotoxic and developmental effects.¹³ A recent report from the Centres for Disease Control and Prevention (CDC) has found that 535.000 children between one and five years old, in the USA, have a minimum of 5 µg Pb dL ⁻¹ of blood. Not much has changed from the results found in 1990.¹⁴ Arguments are being made to clean up areas where the soil is proven to contain high levels of Pb, especially if children often frequent those areas. 


Human population is growing and the world food supply will have to be doubled between 2005 and 2050.¹⁵ The scarcity of soil resources will inevitably force us to cultivate on contaminated areas; metal contaminated land is already exploited for food production to compensate the loss of agricultural surface due to urbanization.¹⁶ All around the world, urban agriculture is booming fulfilling diverse functions including food production, community building, reducing socioeconomic tensions and food millage.¹⁷ Urban allotment gardens have recently been offered by local governments to encourage low income citizens to produce their own food.¹⁸ In New York with long history of gardening 71% of the home garden samples exceed respective Soil Clean-up Objective limits for Pb and As.¹⁹ 52% of crop samples from the Berlin inner city vegetable gardens exceeded standards²⁰ for Pb concentration in food crops.²¹ The citywide study showed that Pb contamination of urban soils is raising a major human health concern that may become a major obstacle for the adoption of urban agriculture.²²

 

Current Remediation methods are ineffective in Pb removal and waste soil 

 

A solution for contamination problems is reclamation and re-use of metal contaminated land by effective, cost-efficientand sustainable (soil preserving) remediation methods. Most countries have made the clean-up and restoration of the contaminated land a priority. Historically, excavation to landfill (dig and dump) has been the solution, offering a quick removal mechanism of a pollution source in the soil. Ever more countries are implementing the legislature with heavy restrictions on landfill. However, other currently available options: soil sealing, separation of contaminated fines, also waste soil. In response, the concept of Gentle Remediation Options (GRO) has emerged. These are techniques that result in no gross reduction of soil functionality as well as risk management.²³ For metal contaminated soils GRO comprises immobilisation and phytoextraction. Up-to-date plant hyperaccumulation of Pb, Cu, Co and Tl remained largely unconfirmed.²⁴ ²⁵ Immobilisation by various additives (phosphates, zeolites, iron and iron oxides etc.) reduces metal plant uptake but does not remove toxic metals from the soil.²⁶ Only a fraction of Pb contaminated soils is treated today due to the lack of efficient and sustainable remediation technologies.


Soil washing and removal of toxic metals with chelants preserves basic soil functions.²⁷ ²⁸ In particular soil washing with ethylenediamine tetraacetate (EDTA) as the most effective, benchmark chelant, was intensively studied for the last two decades by research groups worldwide. However, difficulties with cost effective treatment of vast quantities of process solutions²⁹, absence of feasible EDTA recycling and toxic emissions from remediated soil due to poor EDTA biodegradability and environmental persistence has been an unsolved problem – up to now.



ReSoil® - cost-efficient, effective, soil preserving and emission free remediation technology

We developed an unique soil-washing technology (patent families US 9108233B2 and EP 3153246B) where EDTA and process waters are recycled in an imposed pH gradient and in a closed process loop (Figure 1). To achieve cost-efficiency the ReSoil technology uses inexpensive and waste materials. The alkalinity imposed by Ca-containing base (i.e. lime) destabilizes EDTA chelates with toxic metals.³⁰ ³¹ Consequently, Pb and other toxic metals (Me) are replaced in the EDTA chelate by Ca and toxic metals participate from the reaction as insoluble hydroxides. The chelant in the recycled Ca-EDTA form is much less soil aggressive than commonly used Na-EDTA.³² To shift the chemical equilibrium further towards product formation, we introduced alkaline adsorption of released toxic metals on polysaccharide materials (i.e. waste paper, R3C-OH) for the substitution/ adsorption/ precipitation reaction:³³


2Ca²⁺ + 4(OH)⁻ + 2Me-EDTA + R3C-OH ↔ 2Ca-EDTA + Me(OH)2(s) + R3C-OH-Me(OH)2-(s)


The alkaline part of the process yields more than 90% of recycled EDTA. The remaining EDTA is recycled in insoluble acidic form (H4EDTA, pK4 = 2.7)after addition of sulfuric acid (H2SO4). Excess SO4²⁻ from the acidic and Ca²⁺ from the alkaline part of the process forms insoluble gypsum (CaSO4) which is removed with the remediated soil. The build-up of salty ions and deterioration of process solutions throughconsecutive batches is thus prevented. Gypsum is a plant nutrient source and beneficial soil conditioner.³⁴ Remediated soil is milled to obtain artificial soil aggregates, and then mixed with the cleansed oversized soil material, fertilized and formulated. The post-remedial toxic emissions from soil are mitigated to the levels close orbellow limits of quantification by effective soil rinsing which reduces toxic metals and EDTA concentration in the downstream process waters (Figure 2) and addition of zero-valent Fe (Fe⁰) into the soil slurry (Patent appl. GB 1720126.0) which enables for fast and permanent adsorption of small residual quantities of EDTA and toxic metals chelates. The process is abiotic; poor EDTA biodegradability is not an issue even if exceedingly high chelant concentrations are used in soil washing. Furthermore, Fe⁰ slurry addition simultaneously immobilize As which is a common soil co-contaminant to soil Pb.

 

Figure 1. The ReSoil® technology flowchart. The recovered EDTA and process waters are reused in a closed cycle. The washed soil is rinsed 3-times with recycled process water and at the end with water to compensate for difference in soil moisture between contaminated soil entering and remediation soil leaving the process.

Figure 2. Consecutive rinsing reduces EDTA and toxic metals (i.e. Pb) concentration in the downstream process waters and in the remediated soil.

ReSoil is an ex situ remediation technology – the contaminated soil is excavated and transported to the remediation facility and the remediated soil returned to the site of excavation. Soil washing by EDTA has shown to result in high multimetal (Pb, Zn, Cd, Cu) removal efficiency,³⁵ ³⁶ especially from bioavailable soil fractions.³⁷ ³⁸ Up to 95%  of Pb removal from contaminated soils by EDTA has been documented by other research groups.³⁹ The ReSoil process generates less than 1.1% weight of solid wastes, no liquid wastes are produced. The technology was developed from laboratory / pilot-scale to demonstrational remediation plant with the capacity of 6 tons of soil per day in the city of Prevalje in Meza Valley, Slovenia. The cost of remediation is 136-186 $ per ton of soil, without profit. 


Ecosystem services of EDTA remediated soil


Realistically tended vegetable gardens with EDTA-remediated soil have supported the growth of vegetables,⁴⁰ grasses and horticultural plants (Figure 3).³⁷ Pb uptake by plants was prevented or significantly reduced (Table 1). Remediation with high EDTA doses affected the soil C and N cycles, soil enzyme activities and the structure and abundance of soil microbial populations, especially arbuscular mycorrhizae.⁴¹ ⁴² However, using simple and inexpensive revitalization measures, e.g., addition of compost, healthy un-polluted soil and commercial or indigenous microbial AM inoculum, restored microbial life.⁴¹ ⁴² ⁴³ The chemical and physical properties of remediated soil were mainly preserved.³⁴ ⁴⁴ 

The ReSoil technology is fully consistent with criteria set by GRO concept.

Figure 3. Remediated soil is functional and possess fully restored quality.


Table 1. Concentrations of Pb in vegetables cultivated on original (1585 mg kg⁻¹Pb) and remediated calcareous soil (320 mg kg⁻¹Pb). Data in bold exceed the Pb limit for food safety. LOQ, limit of quantification.