Physicochemical evaluation of coconut shell biochar remediation effect on crude oil contaminated soil

Abstract The effect of coconut shell biochar on pH, cation exchange capacity (CEC), sesquioxides and residual total petroleum hydrocarbon (TPH) of a crude oil contaminated soil was investigated. Raw coconut shells were carbonized in a muffle furnace at 400°C for 2 hours and chemically activated in sulphuric acid solution for 18 hours to produce coconut shell activated carbon (CSAC). The CSAC and crude oil-contaminated soil from Kaduna Refining and Petrochemical Company had their physicochemical properties determined. Six sets of CSAC- soil mixtures containing 1%, 1.5%, 2%, 2.5%, 3% and 3.5% CSAC content to undergo remediation for 36 days. The pH, cation exchange capacity (CEC) and total petroleum hydrocarbon (TPH) degradation increase with higher CSAC content while sesquioxides composition slightly decreases with CSAC addition. A significant reduction in TPH from (2045 to 447) mg/kg was achieved with peak TPH degradation of 78.14% at 3.5% CSAC content. Therefore, CSAC significantly enhances the adsorption and degradation of petroleum hydrocarbons in the contaminated soil.

ABOUT THE AUTHOR John E. Sani The authors are geo-environmental engineers who are specialized in soil improvement and remediation using various techniques.These techniques include microbial induced calcite precipitate and electrokinetic remediation techniques.This article focuses on the use of biological process for the remediation of crude oil-contaminated soil.

PUBLIC INTEREST STATEMENT
This work focused on the physicochemical evaluation of the coconut shell biochar remediation effect on crude oil-contaminated soil, a biological and environmentally friendly method of remediating crude oil-contaminated soil.The result shows total petroleum hydrocarbon (TPH) degradation increases with higher CSAC content, with a peak TPH degradation of 78.14% recorded at 3.5% CSAC content after 36 days of remediation.Field applications of CSAC boost soil fertility by improving biological, physical, and chemical properties of the remediated soil.It is a sustainable and user-friendly remediation technology.

Introduction
Crude oil is a complex and multi-component homogeneous mixture and is composed principally of hydrocarbons including paraffins, naphthenes, and aromatics with minor contents of non-hydrocarbons (e.g., nitrogen, sulfur, and oxygen compounds), resin, and asphaltene (Zhang et al., 2014).Crude oil contamination of soil is a major problem affecting most oil producing nations of the world.Most of the pollution cases involve accidental oil blowouts, seepages and deliberate flushing activities which leaves a thick layer of crude oil over land, vegetation and water surfaces (Uquetan & J, 2017).
Hydrocarbon contaminants are harmful and can affect the quality of groundwater, which becomes unfit for use for a long time (drinking water, irrigation, and different industrial uses).It also poses risk to human health, biological environment, and vegetation (Mariana et al., 2010).Oil spills endanger public health, imperil drinking water, devastate natural resources, and disrupt the economy (Environmental Protection Agency, 1999).
Soil is a critical environmental, ecological, and agricultural resource that must be preserved as construction material and to ensure a sufficient supply of nutritious food for the world's growing population.The Department of Petroleum Resources in 2002 specified TPH limit of 1000 mg/kg as guideline for the toxicity of petroleum hydrocarbon in soil.High concentration of hydrocarbons can alter soil structure, physicochemical and biological characteristics, such as soil organic matter content, bulk density, porosity, permeability, soil respiration and material transfer processes in crude oil contaminated soil (Liang et al., 2012).Osuocha et al. (2013) affirmed soil as the most valuable component of the ecosystem and that environmental sustainability depends absolutely on proper management of the soil.The roles of soil in engineering construction depend significantly on the engineering properties of the soil (Osuji & Akinwamide, 2018).Soil contamination is a major problem facing oil producing regions in Nigeria.Oil pollution on land destroys soil fertility, reduction in crop yields and destroys beneficial soil microbes (Ofomata, 1997).

Remediation
Remediation refers to removing, degrading, or transforming contaminants to harmless or less harmful substances.It includes immobilization of the contaminants, preventing their spreading to uncontaminated areas; toxicity of the contaminants remains unaltered, but the risk they pose to the environment is reduced (US.DOD, 1994;Yuniati, 2018).Adoption of suitable remediation strategy helps in treatment, containment and reclamation of contaminated sites.Examples of remediation technologies are soil removal and replacement, containment, soil flushing, encapsulation, solidification, phyto-remediation, electrokinetic method, and bioremediation.USEPA (2001) highlighted some remediation techniques that can effectively minimize environmental hazards.These techniques include" Natural attenuation or recovery, Physical washing, Excavation, Physical insitu treatment/thermal desorption, Chemical treatment and Biological treatment.
The negative and cost implications of remediation of petroleum contaminated sites using physical and chemical techniques have necessitated the use of biological techniques (Kelechi et al., 2017).Biological remediation is a process in which microorganisms or plants are used to detoxify or remove organic or inorganic compounds from the environment, is a remediation option that offers green technology solution to the problem of environmental degradation (Abioye, 2011).

Biochar and activated carbon applications in soil remediation
The use of biochar and activated carbon for soil remediation is an environmental friendly remediation procedure and offers lots of successes in recovering and restoring the quality of contaminated soil.Biochar is an effective tool for the treatment of contaminated soils because it effectively adsorbs heavy metals and decreases bioavailability and toxin-induced stress to plants and microorganisms (Mackie et al., 2015).Anyika et al. (2015) investigated the impact of biochar on sorption and biodegradation of polycyclic aromatic hydrocarbon in soil due to its environmental friendly nature and observed rapid sorption of polycyclic aromatic hydrocarbon into the biochar, modification of physicochemical properties of the soil, stimulation of microbial activities and degradation of aromatic hydrocarbon.Lehmann (2007) reported that biochar produced from high temperature have large surface area and aromatic-carbon content, which may increase the adsorption capacity (a desirable property for bioremediation) and for carbon sequestration.The properties of biochar, such as yield, ash, specific surface area, pore structure, type and number of functional groups and cation exchange capacity, are affected by methods of preparation (Azubuike et al., 2016;Yang et al., 2019).

Physicochemical properties of CSAC influencing bioremediation
• Higher surface area

A brief description of the sampling site
The sampling site is located 1.2 kilometers away from the main crude oil storage tank within the vicinity of Kaduna Refinery and Petrochemical Company.Inspection and observation of the site show portions of the land along the pipeline have been contaminated.The site had been contaminated by crude oil due to past pipeline spills brought on by failed heavy maintenance equipment.Three samples of contaminated soil were collected from a depth of 0.5 to 1 m into different polythene bags.The soil sample that contained a significant amount of crude oil hydrocarbons of about 2045 mg/kg from the preliminary gas chromatography and mass spectrometry (GC-MS) analysis was thoroughly mixed, air dried, and used for the study.

Materials
The raw materials used for the investigation are crude oil contaminated soil sample obtained from Kaduna Refining and Petrochemical Company (KRPC), located at Chikun, Kaduna, with location coordinate of latitude 10° 21' 42'' N and longitude 7° 27' 30'' E while the coconut shells were collected from Chechenya central market, Kaduna Nigeria.Other materials are 7 set of plastic containers, mixing tray, spatula and Sulphuric acid.

Methods
• Preparation of coconut shell activated carbon (CSAC).
• Physicochemical analysis of CSAC and crude oil contaminated soil using XRF, XRD, SEM, Proximate, FTIRs, and GC-MS analysis.
• Statistical test using analysis of variance (ANOVA).
The experimental sets (CSAC-Soil mixtures) were uniformly mixed with water in a ratio of 20 w/w, studied under atmospheric temperature (Ibrahim et al., 2016), and mixed three times daily while undergoing remediation for 36 days.

Preparation and activation of coconut shell biochar material
A required quantity of coconut shells was collected, washed, crushed, and oven dried at 110°C for one hour.The dried coconut shells were loaded into a muffle furnace to undergo conventional carbonization at a temperature of 400°C for two hours (Efeovbokhan et al., 2019;Lionel & Karunakaran, 2017;Sundaram & Natarajan, 2009).The furnace was purged with nitrogen gas at a rate of 2.5 L/min for 6 minutes to remove trapped oxygen, while a stainless steel pipe connected to the chimney discharged volatilized gases to ensure safety.The coconut shell biochar produced was removed from the muffle furnace, allowed to cool and chemically activated in 5N H 2 SO 4 for 18 hours in accordance to Gawande and Kaware (2017).The activated carbon was thoroughly washed in distilled water, drained, oven dried at 110° C for 3 hours, removed, cooled, ground and sieved through BS sieve 0.3 mm size, and packaged in airtight container.

Physicochemical analysis of CSAC
The physicochemical analysis of the coconut shell activated carbon (CSAC) and the crude contaminated soil were conducted in the laboratories of National

Determination of pH
The pH of the CSAC was determined using ASTMD 3838-05 (2017), standard procedure in which 2 g CSAC was mixed with 100 ml distilled water inside the conical flask and stirred for 1 hour.
Filter paper was used to filter the sample and pH meter was used to measure the pH of the sample.

Iodine number
The quantity in milligrams of iodine absorbed by one gram of activated carbon powder is known as the iodine number.A higher iodine number is an indication that the sample has higher micro-pores (Gawande & Kaware, 2017).The iodine number was determined using ASTM D4607 (2014) in which 2 gm of CSAC was mixed with 10 ml of 5% Hydrochloric acid in conical flask and boiled for 30 seconds.The flask's content cooled to room temperature, 100 ml 0.1N iodine was added, shaken for 30 seconds and its contents filtered through a filter paper.The 50 ml of the filtrate was titrated against 0.1N sodium thiosulphate solution until the yellow color faded completely and its final concentration determined.

Bulk density
Bulk density is weight per unit volume of a material, measured in kg/m 3 .The weight of measuring cylinder was measured and a sample of CSAC was placed in the cylinder and re-weighed.The CSAC sample was dried in an oven at 100°C for 1hour after which the weight of the dried CSAC sample was measured and its bulk density determined as shown in Equation 1.
M1= mass of measuring cylinder in grams M2= mass of measuring cylinder + its contents V= volume of the measuring cylinder in litre.

BET surface area
Brunauer, Emmett, and Teller theory for calculating the surface area of porous powder particles involves the adsorption of gas molecules to the surface of the solid whose surface area is required.
The surface area of the CSAC was determined by using the BET surface area analyzer (Brunauer et al., 1938;Gawande & Kaware, 2017).

SEM analysis of the CSAC
Scanning electron microscopy (SEM) was used to observe the surface morphology of the CSAC samples.The SEM micrograph for the CSAC shows visible pores on the CSAC as a result of activation process.

Determination of oxide and elemental compositions
100 g each from the contaminated and CSAC remediated specimen were taken to a commercial biochemical laboratory to determine the oxide and elemental composition using X-Ray Fluorescence machine, while X-Ray diffractometer was used for the qualitative and quantitative analysis of minerals in the crude oil contaminated soil and CSAC remediated soil.

Determination of total petroleum hydrocarbon
Solvent extraction and the GC-MS analytical procedure in accordance to Bada et al. (2019) were used to determine the total petroleum hydrocarbon (TPH) of the crude oil contaminated soil.In a clean bottle, 10 grams of the crude oil contaminated soil sample was weighed, and 25 ml of dichloromethane was added; the mixture was placed in a mechanical sieve shaker at speed of 350rpm for 3 hours.This procedure was repeated twice and extract was collected into a beaker, the extract was concentrated to 5 ml in a steam bath and passed through a pipette containing anhydrous Sodium sulphate on a glass wool to remove impurities and moisture.The extract was analyzed in a GC-MS system to quantify TPH.The percentage of TPH degraded was determined as shown in Equation 2.

FTIRs analysis of the crude oil contaminated soil
FTIR spectroscopy of the soil sample was conducted to identify the functional groups of hydrocarbons present in the soil sample.The FTIR chat is presented in Figure 1 and the FTIR result is presented in Table 1.

XRD and XRF analysis of the crude oil contaminated soil
The result of X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis showing minerals and oxides composition in the crude contaminated soil are presented in Tables 2 and 3 respectively.

Properties of coconut shell activated carbon
The results of proximate analysis (ASTM D3172, 2007) and XRF analysis of oxides in the CSAC are shown in Tables 4 and 5.The surface area of the prepared CSAC is 667 m 2 /g and within the range (82.77 to 715.16) m 2 /g reported by Buah and Kuma (2012) for adsorbent.The iodine number of the CSAC is 877 mg/g, an indication of high adsorption capacity reported by Gawande and Kaware (2017).The high CEC of 51.62 Cmol/kg indicates the capacity of the biochar to adsorb cationic nutrients and metal contaminants while the BET surface area determines biochar's ability to retain water, gases, and organic molecules (Guo et al., 2020).

SEM analysis of CSAC
Scan electron microscope images were taken to observe the pore structures of the coconut shell activated carbons.The result indicates that CSAC have high surface area due to well-developed pores.A macro porous structure in biochar is potentially important to water holding capacity and adsorption of pollutants in soil and solutions (Angin, 2013;Gao et al., 2013).The micrographs of the activated carbon are shown in Figure 2(a,b).

X-Ray diffraction of CSAC
XRD pattern of coconut shell activated carbon exhibiting peak around 2θ is shown in Figure 3.The diffraction peak of 2θ observed at diffraction angles of 29.5°, 34.6° and 39.4° gave a pattern that is attributed to the presence of silicate minerals, iron ore, and quartz (IkhtiarBakti & Gareso, 2018).
The XRF analysis of sesquioxides composition and summary of analyzed GC-MS result showing the percentages of total petroleum hydrocarbon degraded are shown in Tables 6 and 7   The result of pH (See Figure 4) increases with higher CSAC content (from 5.9 to 7.4), an indication that CSAC reduces the acidity/toxicity of the crude oil contaminated soil.The increase in soil pH is attributed to the higher pH (alkalinity) of CSAC resulting base saturation, increase in surface charge enabling CSAC to attract polar organic molecules and ionized organic contaminants (Guo et al., 2020).The result is in conformity with the findings of Zhang et al. (2016) and Rabileh et al. (2015).The result of the one-way analysis of variance (ANOVA) test on the pH (See Table 8) (F CAL = 38.07>FCRIT = 4.75) shows CSAC has significant effect on the pH of the remediated soil.
The variation of CEC with CSAC content is shown in Figure 5.The result shows CEC increases with higher CSAC content (from 11.1 to 25.5) Cmol/kg.This result may be attributed to the presence of organic materials of variable charge that can potentially increase CEC and base saturation of soil as previously reported by Glaser et al. (2002) and Dume et al. (2016).It also indicates abundance of surface functional groups which determines biochar capacity to stabilize heavy metals through sorption (Guo et al., 2020), an essential mechanism for strength gain in soil.One-way analysis of variance (ANOVA) test on the CEC (See Table 8) (F CAL = 38.07>FCRIT = 4.75) shows CSAC has significant effect on the CEC of the remediated soil.The variation of sesquioxides (SiO 2 , Al 2 O 3 and Fe 2 O 3 ) with CSAC contents is shown Figure 6.The sligth reductions in sesquioxides with higher CSAC content may be attributed to sorption and precipitation of iron and aluminium,hence, limiting their for pozzolanic reaction.Therefore, strength gain depends basically on the CEC, elevated pH/surface charge of CSAC resulting to covalent bonding and stabilization of contaminats ( (Essington, 2003;Sparks, 2003;Sposito, 2008).
The result of TPH degradation is shown in Figure 7.The result shows TPH degradation increases with higher CSAC content and peak TPH degradation of 78.14% recorded at 3.5% CSAC content.This implies a significant reduction in initial TPH, from 2045 mg/kg to residual TPH of 447.04 mg/kg after 36 days of remediation.This result is attributed to rapid adsorption and degradation of crude oil contaminants enhanced by presence of micro pores, surface area, elevated pH/surface charge and CEC.The result is in conformity with previous findings reported by Zimmerman et al. (2004), Cornelissen et al. (2006), Brandli et al. (2008), Rhodes et al. (2008) and Ibrahim et al. (2016).

Field application of CSAC
Coconut shell activated carbon can be applied in crude oil-contaminated sites by using mechanized tools such as excavators, bucket mixing, rotary mixing tools, augers, and multiple auger tools for homogeneous surface and deep mixing of CSAC with soil in large field applications to achieve the following remediation results: (i) Significant reduction in soil toxicity due to the alkaline pH of the CSAC.(ii) Reduction in the total petroleum hydrocarbon (TPH) content of the crude oil-contaminated soil.(iii) As the soil's cation exchange capacity rises, heavy metal adsorption from the polluted soil is improved.(iv) The surface area and pore volume in CSAC serve as residences for beneficial soil microbes, thereby improving soil fertility.The remediated soils become suitable for agricultural and engineering applications.(v) Coconut shell activated carbon is an effective adsorbent material that absorbs leached contaminants from the soil, thus protecting groundwater.(vi) The moisture holding capacity of the remediated soil is increased due to the CSAC's high surface area, pore volume, and adsorption mechanisms.(vii) The CSAC adsorption mechanisms involving surface functional groups, surface adsorption, pore filings, higher pH, and CEC provide efficient soil remediation with significant improvements in the physical, chemical, and biological properties of the remediated soil (Guo et al., 2020).

Conclusion
A coconut shell activated carbon was prepared, adsorbent properties and its remediation performance on crude oil contaminated soil were investigated.The following conclusions were established based on the results of the findings: (a) The coconut shell activated carbon prepared has adsorbent properties: pH of 8.7, CEC of 51.62 Cmol/kg, BET surface area of 667 m 2 /g, iodine number of 877 mg/g, and volatile matter of 28.2 % suitable for sorption, remediation and stabilization of inorganic/organic contaminants in soil.
(b) TPH degradation increases with higher CSAC content given rise to peak TPH degradation of 78.14%at CSAC content of 3.5 %.
(c) A significant reduction in total petroleum hydrocarbon (TPH) from 2045 mg/kg to residual TPH of 447.04 mg/kg below critical TPH toxicity limit of 1000 mg/kg specified by Department of Petroleum Resources "DPR" (2002) was achieved.
(d) The effect of CSAC on sesquioxide is statistically insignificant, though slight reduction in sesquioxide composition with higher CSAC content could impact negatively on strength properties of the soil.
(e) The result of statistical test (ANOVA) on the CSAC remediated soil shows pH, CEC and TPH significantly affected by CSAC.Therefore, CSAC significantly enhances the adsorption and degradation of crude oil contaminated soil.

Recommendations
The following recommendations are given based on the research findings: (a) Adoption of higher CSAC content is recommended since TPH degradation increases with higher CSAC content.
(b) Remediation under suitable atmospheric condition and frequent mixing/homogeneous mixture is recommended to achieve significant reduction in residual TPH.
(c) Application of CSAC with verified physicochemical and suitable adsorbent properties is for optimum remediation performance.

Suggestion for further investigation
(a) Geotechnical evaluation of coconut shell biochar remediated crude oil contaminated soil for use as granular subbase and subgrade in road pavement.
(b) A comparative study of coconut shell biochar stabilization effects on lateritic and nonlateritic soil.

Figure 1 .
Figure 1.FTIR chart of the crude oil contaminated soil.
Figure 2. (a) SEM of CSAC at X 800 (b) SEM of CSAC at X1000.
Figure 4. of pH with CSAC content.
Figure 6.Variation of sesquioxides with CSAC content.

Figure
Figure 7. Percentage of TPH degraded.
Steel Raw Material Exploration Agency (NSRMEA) located at Malali village, Kaduna State and Chemical Engineering Department, Ahmadu Bello University Zaria, Kaduna Nigeria.Proximate analysis of CSAC to determine the percentage of fixed carbon, volatile matter, moisture content and ash content was done in line with ASTM D3172 (2007) standard.

Table 8 . Summary of analysis of variance (ANOVA) test on pH, CEC, and sesquioxides
Crit : Significant effect; F Cal <F Crit : Insignificant effect)