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  • Biochar’s positive effect on contaminated soils

    Overview

    Over the years there have been a number of studies into the effect biochar has on remediating soils that are contaminated with heavy metals such as cadmium, lead and zinc, dangerous chemicals, such as arsenic, pesticides and organic pollutants.

    Below is a summary of five such studies, their methodologies and findings.  We’ve also included some highlights if you want a quick summary to read.

     

    Highlights
    • Biochar can effectively reduce soluble cadmium and zinc from contaminated soils by up to 300 times (cadmium) and 45 times (zinc). Once sorbed onto the biochar the metals aren’t released following further washing with water (Beesley and Marmiroli, 2011)

     

    • Biochar can reduce the uptake of arsenic (As) by plants growing in As contaminated soils. Care is needed as the pH of the soil can rise with biochar additions, which can make As more soluble (Beesley et al. 2013). In their trial using tomato plants, As was increased in soil solution from biochar applications, however root and shoot As concentrations were lower when biochar was added and there was no increase in As content of the fruit.

     

    • Biochars can immobilise zinc and copper from contaminated soils, and also pesticides and Persistent Organic Pollutants (POP’s). It is recommended to apply organic amendments such as compost as well as biochar, since biochars can also immobilise major nutrients such as N and P (Beesley et al. 2011), particularly biochars which have high cation exchange capacities.

     

    • Biochars can adsorb up to 500 times more POP’s than soil alone (reference soil containing 5% organic matter). This is due to high surface area, high cation exchange capacity and long residence time in soil. Heavy metals are precipitated onto the surfaces of biochars due to rising pH (copper, zinc, lead and cadmium) forming insoluble phosphates, but not arsenic, as this is an anionic metal (Hilber et al. 2017)

     

    • Further ways of biochar immobilising heavy metals include oxygen functional groups e.g. Cr 6+ is soluble and can be reduced to Cr 3+ which is far less mobile (Paz-Ferreiro et al. 2014). Biochar doesn’t reduce the total levels of heavy metals in soils, it reduces the bioavailability to plants and mobility in soils (also noted by Beesley et al., 2013). Phytoremediation consists of growing plants which can actively take up heavy metals, these can be harvested to reduce the level of contaminants in the soil. Although not tried yet, the authors Paz-Ferreiro et al. suggest a combination of biochar addition and phytoremediation as a good option for polluted soils.

     

    Case studies

    Beesley and Marmiroli (2011) ran a column leachate experiment using contaminated soil from Kidsgrove, Staffordshire, containing cadmium and zinc.

    Columns of soil were leached with water for 5 weeks and the eluate run through columns of biochar (hardwood -derived, pH 9.9, total organic carbon 53%).

    Following leaching for 5 weeks and adding the leachate to the biochar columns, the biochar columns were further leached with deionised water for a further 3 weeks to study whether cadmium and zinc were released.

    The biochar reduced the cadmium in the leachate by 300 times and the zinc by 45 times in the experiment.

    The authors concluded that the porous nature of the biochar effectively adsorbed and retained the heavy metals. Arsenic is relatively insoluble in water and low levels were eluted from the contaminated soil columns. Soluble metals are likely to cause the most environmental impact, particularly cadmium.

    Beesley et al. (2011) reviewed the role of biochar in remediation of contaminated soils in a number of studies.

    Generally, biochar additions immobilise copper and zinc and reduce leaching of these heavy metals (soil column experiments). In trials where biochar was added to soils in the field rather than the laboratory instances were found of reduction of available cadmium, lead and zinc with 10% biochar addition.

    One conclusion by the authors was to add organic amendments as well as biochar, particularly biochars with high Cation Exchange Capacity, which are good at immobilising heavy metal contaminants but also can lock up nutrients as well such as nitrogen and phosphate.

    Biochars can also effectively remove persistent organic pollutants and some pesticides from soils as well. A number of studies are cited to support this.

    Beesley et al. (2013) used an arsenic (As) contaminated soil from mine workings near Madrid, Spain (As content 6000 mg/kg), the soil had a low pH of 5.0.

    Biochar from orchard prunings (pH 10.0) was added to soil at 30% by volume and the mix put into 1 litre pots and tomatoes grown in the mix. Some plants received additional NPK liquid fertiliser. Pore water was collected, it is known that increasing pH will mobilise arsenic, this was found in pore water samples from the pots as the trial progressed.

    However, the AS concentration in the roots and shoots was much lower with the biochar addition (3 x less in roots, 5 x less in shoots), when fertiliser P was added, root uptake of AS was increased, but was still lower than the plants growing in the unamended soil .

    The level of AS in the fruit was very low both with and without added P fertiliser for the two biochar treatments.

    The authors concluded that care is needed if adding biochar to AS contaminated soil of low pH as this may release soluble As. However, the biochar itself assisted in the reduced uptake of AS by the tomato plants.

    Paz-Ferreiro et al. 2014 also reviewed remediation of soils using biochar. They postulated that oxygen functional groups on the surface of the biochar were able to stabilise heavy metal cations as well as the pH effect, which was the most likely cause of immobilisation.

    The paper goes in some detail into phytoremediation (growing plants which actively accumulate heavy metals) and suggests a combination of phytoremediation and biochar applications may be the way forward to manage polluted soils.

    Hilber et al. (2017) reviewed many papers on biochar and its effects on POP’s and heavy metals in soils.

    Biochars (21 tested) can absorb around 500 x more persistent organic pollutants that a reference soil containing 5% organic matter and 20 times more than average organic carbon.

    This is due to their high surface area, high Cation Exchange Capacity and long residence time in soil i.e. not degraded over time. Heavy metals precipitate as phosphates on the surface of biochars due to the rise in pH e.g. copper, zinc, lead and cadmium.

    However, metalloids and non-cationic negatively charged metals such as arsenic aren’t precipitated out on biochar surfaces.

    The authors comment that there are very few studies in the field compared to laboratory trials looking at biochar bioremediation. One study in China on paddy rice soil contaminated with cadmium showed that biochar added to the soil (typically 0.5 to 2.0% additions) was able to reduce cadmium uptake by between 57% and 86% in rice grains.