COMBINED EFFECTS OF PLANT CULTIVATION AND SORBING

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COMBINED EFFECTS OF PLANT CULTIVATION AND SORBING CARBON AMENDMENTS ON FREELY DISSOLVED PAHs IN CONTAMINATED SOIL Patryk Oleszczuk, Magdalena Rakowska, Thomas D. Bucheli, Paulina Godlewska, and Danny Reible Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06265 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Environmental Science & Technology

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COMBINED EFFECTS OF PLANT CULTIVATION AND SORBING

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CARBON AMENDMENTS ON FREELY DISSOLVED PAHs IN

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CONTAMINATED SOIL

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Patryk Oleszczuk1,2*, Magdalena Rakowska2, Thomas D. Bucheli3, Paulina Godlewska1, Danny D.

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Reible2

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1Department

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Square, 20-031 Lublin, Poland

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2Civil,

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USA.

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3Agroscope,

of Environmental Chemistry, Faculty of Chemistry, 3 Maria Curie-Skłodowska,

Environmental, and Construction Engineering, Texas Tech University, Lubbock, TX 79409,

Environmental Analytics, Reckenholzstrasse 191, 8046 Zürich, Switzerland.

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* corresponding author: Patryk Oleszczuk, Department of Environmental Chemistry,

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University of Maria Skłodowska-Curie, 3 Maria Curie-Skłodowska Square, 20-031 Lublin, Poland,

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tel. +48 81 5375515, fax +48 81 5375565;

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e-mail: [email protected]

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Abstract

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We report freely dissolved concentrations (Cfree) of PAHs in soils amended with 2.5% biochar and

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AC during a long-term (18-months) field experiment. The study evaluates also the impact of

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different plants (clover, grass, willow) on Cfree PAHs. The cumulative effect of treatments on

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nitrogen and available forms of phosphorus, potassium and magnesium is also assessed.

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The direct addition of biochar to soil did not cause any immediate reduction of the sum of 16 Cfree

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PAHs, while AC resulted in a slight reduction of 5- and 6 ring compounds. The efficiency of

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binding of Cfree PAHs by biochar and AC increased with time. For biochar, the maximum reduction

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of 4-6-ring PAHs (18-67%) was achieved within 6 months. For 2- and 3-ring PAHs a gradual

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decrease of Cfree was observed and reached 60-66% at 18 months. AC proved better in reducing

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Cfree PAHs than biochar, though for 2- and 3-ring PAHs the differences in AC and biochar

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performance were smaller than for 4-6-ring PAHs. After 18 months, a significantly lower content of

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Cfree PAHs was observed in the soil with plants compared to the unplanted soil. Except for

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potassium, AC or biochar did not negatively impact nutrient availability.

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Keywords: PAHs; remediation; activated carbon; biochar; plants; phytoremediation; bioavailability

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INTRODUCTION

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Remediation

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technologies

for

PAH-contaminated

soils

include

solvent

extraction,

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bioremediation, phytoremediation, chemical oxidation, photocatalytic degradation, electrokinetic

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remediation and thermal treatment.1 The greatest potential risk management benefit of in situ

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amendments with carbonaceous materials on large contaminated sites is due to containment and

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long-term exposure reduction of both organic and inorganic contaminants. High affinity of

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contaminants for biochar or AC provides a reduction of freely dissolved PAH concentrations (Cfree)

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in sediment or soil porewater, which in turn is the most available fraction relevant for exposure,

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bioaccumulation, and effects.2,3 Hence, the addition of biochar or AC reduces mobility of HOCs

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and is expected to reduce adverse effects on organisms and the environment.4–7 The use of carbon

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sorbents to immobilize contaminants has been demonstrated in freshwater and marine sediments.6–

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PAHs up to 99.8%, depending on PAH hydrophobicity.9,12–14 However, other reports have shown

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that AC may cause adverse effects in soil organisms.15

It has been shown that a 2 to 5% addition of AC to sediment reduces the content of bioavailable

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In recent years, the potential of contaminant immobilization has also been explored in soils

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amended with other carbonaceous sorbents, including biochar.5,16–19 Although, biochar

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effectiveness in reducing contaminant availability in soils is lower compared to AC,4,20 it may

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positively affect plant growth and development as well as soil enzymatic activity.21,22

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To date, the performance of biochar or AC in soils have been studied mainly at a small scale

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(laboratory or greenhouse pot experiments), with exposure time of 1-15 weeks.5,17,18,23 Long-term

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effects of carbonaceous amendments addition to soils under field conditions and for extended time

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(i.e. 18 months) have been studied less frequently.17,20 The short term studies demonstrated up to

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80% reduction of bioavailable (freely dissolved or bioaccessible) PAHs after biochar addition to

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soils5,18,23 and significantly reduced bioaccumulation in plants5,23 and soil invertebrates.18 Biochar

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was reported to reduce soil toxicity, although the effect strongly depended on the soil type and ACS Paragon Plus Environment

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characteristics.15 However, it is still uncertain to what extent Cfree changed for individual PAHs

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throughout the experiment and how much time a system requires to reach steady-state under field

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conditions.

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In the literature, there is also a lack of long-term comparative studies between biochar and AC

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to immobilize PAHs under natural field conditions. Biochar requires more time than AC to bind

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individual PAHs,16 which is an important factor in designing efficient remediation scenarios.

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Moreover, the sorption properties of biochar change with time,24 which may also determine the

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(net-) rate of ad- and (possibly) desorption of contaminants.

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Many sites have abundant vegetation, therefore from a practical point of view, understanding

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the effects of plants on the content of Cfree PAHs, in relation with biochar or AC application is

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advantageous. It has been shown that plants may increase bioavailable PAHs in soil.25,26

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Conversely, plant rhizosphere may affect the degradation of PAHs and thus stimulate the growth of

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microorganisms.27 However, biochar or AC addition to soil may also reduce the availability of

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nutrients necessary for plant growth, and thus limit the applicability of this remediation technique.

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In both cases, an appropriate selection of plants can optimize the efficiency of elimination of

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bioavailable PAHs. Analysis of such secondary effects of biochar or AC amendment has been

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mainly limited to dissolved organic carbon (DOC) measurements,17,19,20 whereas the accessibility of

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available forms of nutrients after biochar or AC application can play a key role in overall method

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effectiveness. To date, only one study evaluated the influence of AC on nutrient concentration in

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leachates from AC-amended soils and did not observe AC to significantly affect nutrient content.20

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Nevertheless, biochars may have diverse capacities for nutrients, therefore different biochars and

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treatment scenarios need to be explored.

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The primary aim of this study was to compare the efficiency of AC and biochar in reducing Cfree

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PAHs in soil in a long-term field experiment. A second aim was to determine the effect of different

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plants on the changes of PAH Cfree in a biochar- or AC-amended soil as well as the effect of AC and

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biochar on the content of available forms of nutrients. Cfree PAHs were accurately determined at the


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beginning of the experiment and after 6, 12 and 18 months using 76 µm polyoxymethylene (POM)

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passive samplers. We believe this to be the first long-term study exploring the interplay of

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carbonaceous materials and plants crucial for designing effective soil remediation scenarios.

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MATERIALS AND METHODS

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Adsorbents. Biochar (BCW) and AC were used in the field experiment. The BCW was provided by

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Fluid SA (Poland) and produced from dried willow (Salix viminalis). The slow pyrolysis

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temperature was set from 600 to 700 °C. The coal-based AC was purchased from POCH (CAS:

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7440-44-0, Poland). BCW and AC were evaluated for pH, carbon, hydrogen and nitrogen content,

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the total organic carbon content (TOC), black carbon content (BC), N2 adsorption and specific

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surface area (SBET). Details of these methods are provided as Supporting Information (SI). The

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adsorption of representative 3-, 4- and 5-ring PAHs (phenanthrene, pyrene, benzo[a]pyrene) to pure

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AC and BCW was also evaluated (detailed information about sorption experiment is presented in

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SI).

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Field experiment. The field experiment was performed near Chełm, Poland (51°11'49.7"N

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23°15'01.2"E). Contaminated soil was transported from the location of a coking plant in Dąbrowa

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Górnicza, Silesia, Poland and homogenized using a mechanical (concrete) mixer before placing in

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twelve 2 m (wide) x 2 m (long) x 0.2 m (deep) plots. The experiment was carried outdoors in an

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agricultural station under the influence of environmental conditions (e.g. rainfall, sun exposure,

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etc.). No special treatment (no pesticides, no agricultural practices, no fertilization) was applied.

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The study employed three mesocosms (Fig. S1): (1) control (without amendments), (2) treatment

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with biochar – BCW and (3) treatment with AC. To explore the effect of diverse plants on Cfree

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PAHs, parallel systems comprising willow (Salix viminalis), mix of grasses (Lolium perenne L.,

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Lolium multiflorum, Festuca arundinacea and Dactylis glomerata L.), white clover (Trifolium

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repens L.) and no plant addition were included in the field trial. Willows are used for short-rotation ACS Paragon Plus Environment

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coppices because they grow rapidly (2–3 m per year), are easy to cultivate, and yield high biomass

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when planted at high densities. Willows can be used for different purposes, for example as carbon

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dioxide neutral biofuel, and are frequently applied in phytoremediation.28,29 Perennial ryegrass is an

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important pasture and forage plant, and is used in many pasture seed mixes. In fertile soil it

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produces a high grass yield, is frequently sown for short-term ley grassland, often with red (T.

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pratense) or white clover (T. repens). The plants were sown at an approximate rate of seeds of 20

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kg per 2.5 acres at the beginning of the field experiment in April 2014. Twelve plots were prepared

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and eight were amended in early spring 2014 with 2.5% of AC or BCW based on soil dry weight.

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The amount of sorbents were selected based on experiences with earlier own research,16 and typical

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numbers from the literature.30 AC or biochar were added during spring tillage operations before

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sowing and were mixed with soil by a rotatory tiller. Soil samples were collected during the field

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trial in 2014 and 2015, initially every 3 months and then every 6 months (i.e. April 2014, July 2014,

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October 2014, April 2015, October 2015). Non-amended and AC or biochar-amended soil was

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sampled at 0–20 cm depth with a stainless-steel corer (5 cm ID and 60 cm long). Six independent

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sub-samples were taken from each plot during individual sampling events. Sub-samples (six from

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each plot) were mixed together to obtain the composite sample from each plot. The samples were

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transported to the laboratory, air dried in an air- conditioned storage room (about 25 °C) for 9

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weeks (in darkness), manually crushed, and sieved (60% to the Σ16 PAHs) (Table

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3). The mass transfer from contaminated soil to AC particles can be faster for the smaller PAHs

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than for the larger ones because of higher mobility and water solubility of the smaller PAHs41

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suggesting that, as in the control soils, the release from the remaining carbon in the system was able


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to sustain Cfree initially. It is likely that the relatively low dosage of BCW (2.5%) was insufficient in

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reducing available PAHs given the comparatively large amount of natural organic matter and black

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carbon in the soil as observed by an earlier study.18 Larger effectiveness in reducing Cfree has been

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reported in soils amended with biochar doses >5%.5,16,23 It should also be emphasized that in the

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present study for 2- and 3-ring PAHs no significant differences were observed in the Cfree content

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between the AC- and biochar-amended soils (Fig. 2), which likely indicates that the soil organic

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matter was able to sustain porewater concentrations of these species despite likely differences in

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sorption between AC and biochar.

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Effective and immediate reduction of Cfree (i.e. 92.6 - 96.6%) was observed in 4-6-ring PAHs, in

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the AC application scenario. For the same compounds, application of BCW resulted in 18.5% to

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22.4% (Fig. S4) decrease in Cfree only, most likely due to the weaker affinity of PAHs to BCW as

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compared to AC (Fig. S2). More effective reduction of heavy molecular weight (HMW) PAHs as

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compared to low molecular weight (LMW) compounds in AC systems was also observed in other

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studies.16,47 This may be due to the fact the heavy molecular weight PAH were present in the

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porewater at much lower concentrations compared to more labile 2-3 ring compounds as well as the

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relatively slow desorption of the heavier compounds. The desorption from native soil organic

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matter of heavy PAHs is relatively slow, whereas adsorption onto AC is relatively fast, leading to a

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depletion of these compounds in the aqueous phase (low Cfree), which seems to be more prominent

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for heavy than for light PAHs. Typically, the nonlinear sorption onto carbonaceous materials lead to

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much greater effective partition coefficients at lower concentrations.

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The porewater concentration results for Σ16 Cfree PAHs in both biochar- and AC-amended soils,

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showed a systematic decrease with time (Fig. 1A). After 18 months, the content of Cfree PAHs

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decreased in biochar and AC systems by 58.8 and 76.3%, respectively. When normalized to Cfree in

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control soil the reductions in AC and biochar systems were 59.5% (biochar) and 80.1% (AC). The

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largest reduction in 4-6 rings PAH was observed in AC during the initial stage of the study and only

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minor further reductions were observed with time (Fig. S4). However, over the 18-month period


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significant changes were found for 2- and 3-ring PAHs, in both biochar and AC exposures (Fig. S4)

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relative to unamended controls, as well as for 6-ring PAHs in the system with biochar.

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To date, this has been the first comparative study between AC and biochar used to immobilize

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Cfree PAHs conducted under natural conditions over a period of nearly two years. So far,

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comparative studies had been performed for a period not exceeding 60 days in laboratory

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experiment and revealed significant differences in Cfree reduction between AC and biochar.18 Our

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results show that for 2- and 3-ring PAHs, longer contact time is required between the contaminated

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soil and biochar. It is plausible that a decreasing trend could still be observed (even after 18

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months) for these compounds. In case of the heavier compounds, steady state was reached already

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after 6 months from biochar incorporation (4-6-ring PAHs) or immediately after AC incorporation

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(5- and 6-ring PAHs). The observed reduction of PAHs, after application of both biochar and AC,

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was comparable to that found in the previous studies concerning soils in the lab and the field

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scale.5,16,18,20,23,48

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The effect of plants on Cfree in sorbent amended systems. The changes in Σ16 Cfree PAHs in the

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experiment involving growth of different plants are shown in Figures 1B-D. Additional plots

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showing porewater changes for Σ16 PAH are presented as Fig. S5 (SI). Plots for individual PAH

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groups with different plants, in control soil and in the biochar- or AC-amended soil are shown in

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Fig. S6 (SI).

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The effects of the plants on total Cfree PAHs did not differ statistically (ANOVA, P≥0.05)

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between different sampling points in the control soil and for majority of time points in the biochar-

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amended soil (Fig. 1A, C, D, Fig. S5). However, significant differences were found in AC amended

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plots, where Cfree reductions can be primarily explained by PAH sorption to AC. Statistically

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significant differences (ANOVA, P≥0.05) were noted for willow during all sampling events, clover

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after 3, 6 and 12 months, whereas for grass only after 12 months. Depending on the sampling point,

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the porewater concentration decreased by 17 - 54% in the soil with willow and by 31 - 42% in the

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soil with clover in comparison to the unplanted soil (Fig. 1A, C, D; Fig. S5).


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In the case of individual PAH groups (Fig. S6-S9), significant differences (ANOVA, P≤0.05) in

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Cfree content could be observed between the experiments with and without plants and between

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sampling events. We could not unambiguously demonstrate that the specific plants have a clear

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influence on the content of Cfree PAHs in the soils. There were no clear trends with time despite the

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statistical differences between sampling periods. The ultimate changes were most clear by

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comparing the initial and the final sampling period, for details see Figure S4.

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For most of the PAH groups no significant differences were observed between the Cfree PAHs at

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the last sampling date in the unamended soil with plants relative to the unamended and unplanted

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soil (Fig. S6, first column). However, for 5-ring PAHs the Cfree values increased from 31 to 97%

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(grass

COMBINED EFFECTS OF PLANT CULTIVATION AND SORBING

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