Chemical characteristics of groundnut and sheanut shell biochars as adsorbents and soil conditioners in the era of ecological sustainability

This study investigated the inf luence of pyrolysis temperatures on characteristics of groundnut and sheanut shell biochars as potential adsorbents and soil conditioners. Groundnut and sheanut shell biochars were produced at pyrolysis temperatures of 350 ± 5 °C and 700 ± 5 °C using muff le furnace. The chemical characteristics of the biochars were analysed, potential contamination and ecological risk were determined based on the metal enrichment index and potential ecological risk index (PERI). pH values of t he biochars ranged f rom 9.42 to 10.23 and 662.33 to 3206.67 μS/cm for electrical conductivity. The total compositions of carbon and nitrogen for GB350, GB700, SB350 and SB700 ranged f rom 58.13% to 70.23% and 0.45% to 1.37%, respectively. The minerals composition of GB350, GB700, SB350 and SB700 ranged f rom 12944.92 to 20873.30 mg/kg for potassium, 192.24 to 410.72 mg/kg for sodium, 3567.98 to 13451.83 mg/kg for calcium and 1150.33 to 3414.34 mg/kg for magnesium. The pH of the biochars is found to be alkaline which upsurge with increasing pyrolysis temperature. Concentrations of nutrients such as calcium, potassium, magnesium and phosphorus diverse in groundnut shells feedstocks due to the pyrolysis conditions. The groundnut and sheanut shell biochars can increase essential nutrients such as nitrogen, phosphorus, and potassium in soil, which are conducive to growth of plant. The availability of phosphorus in the biochars make it phosphorus rich and can be used as slow-release fertilisers. The potential toxic metals in the groundnut and sheanut shell biochars have values that suggested low contamination and less potential ecological risk making the biochars ecofriendly. Groundnut and sheanut shell biochars can be used in f ields as an adsorbent and a soil amendment based on its chemical characteristics.


INTRODUCTION
Biochar is produced through slow pyrolysis process where organic material is heated in a complete or almost complete oxygen f ree environment to 300 °C to 700˚C . Biochar ability to adsorb pollutants varies depending on target pollutant and its physico-chemical properties (Ahmad et al., 2014a). Adsorption process is inf luenced by numerous f actors that play a vital role including the capacity of adsorption, surf ace area and mechanical stability (Khan et al., 2020).
Biochar properties are more inf luenced by the f eed stocks type or biomass than pyrolytic temperature. Hence, in designing a biochar for agricultural purpose, f eed stock and pyrolysis temperature are the key f actors to be caref ully considered (Muhammad and Abdul, 2020). Temperature of pyrolysis has great inf luence on the structural, morphological, elemental and characteristics of biochars (Kołodyn´ska et al., 2012). It is obvious that morphological and physico-chemical characteristics of biochars also depend on the nature of f eed stock. Theref ore, the main challenge is how to predict and produce a good biochar that will be agronomically acceptable, benef icial to soil and ecologically sustainable f rom any known f eed stock by any given charring technology and production conditions (Hassnen et al., 2020).
Biochar production is a cost-ef f ective approach f or recycling of waste due to the increasing price of disposal of waste (Pariyar et al., 2020). The adaptation of this new method has aided f armers to better choose mineral and organic f ertilisers and corresponding agronomic operations, so the soil can increase water retention capacity and provide higher yields, which results in enhanced water retention during droughts or extreme rainf alls, overall lessening the cost (Maroušek et al., 2020). Soil f ertility can be improved by biochar through the enhancement of the availability of essential nutrients f or instance carbon, nitrogen and phosphorus (Zhang et al., 2016).
Biochar has been generally ref erred as an ecof riendly soil amendment however harmf ul components (dioxins, environmentally persistent f ree radicals (EPFRs), heavy metals, perf luorochemicals (PFCs) and polycyclic aromatic hydrocarbons (PAHs)), may be produced owing to the preparation methods, preparation conditions, and unsuitable selection of f eedstocks (Xiang et al., 2021). As a result of, its various interactions and potentially detrimental components with the environment, some researchers and scientists have taken interest in the negative ef f ects of biochar on the environment (Cui et al., 2021). Phytotoxicity of biochar research is mostly on germination experiments, which have some inadequacies, such as unclear internal mechanism, long experiment times, and other uncontainable f actors (Malf atti et al., 2021). Godlewska et al. (2021) studied biochar potential environmental risks in soil (a single environmental medium); nevertheless, the biochar potential hazards on the atmosphere and waterbodies, in addition to the ef f ects on diverse media are limited. Theref ore, the overall potential risks of biochar application in soil, water, and the atmosphere must be comprehensively studied to determine the corresponding occurrence, detection, assessment, and avoidance measures of these risks.
Chemical characteristics inf ormation on biochar will help in its application in the environment, agriculture, nanotechnology and industry. Groundnut and sheanut shell in Ghana are usually burnt or lef t on the f ield to rot af ter harvesting. This can be recycled into biochar that have the potential of being used as an adsorbent f or contaminant removal or immobiliser and as soil conditioners. Hence, this study investigated the inf luence of pyrolysis temperatures on groundnut and sheanut shell biochars and judged the chemical, elemental and nutrient composition that could serve as predictors of their suitability as potential adsorbent and soil conditioners. The potential of each f eed stock to adsorb metals and as soil conditioners are discussed with respect to chemical properties, and conclusions are drawn on their suitability.

Feed stocks and pyrolysis condition of biochar
Groundnut and sheanut shells were used to produce biochars (GB350 and GB700: groundnut shell biochar produced at pyrolysis temperatures of 350 ± 5 °C and 700 ± 5 °C, respectively; and SB350 and SB700: sheanut shell biochar produced at pyrolysis temperatures of 350 ± 5 °C and 700 ± 5 °C, respectively) in the Agricultural Sub-sector Improvement Programme (AgSsIP) Laboratory in the University f or Development Studies, Nyankpala Campus. The groundnut and sheanut shells were collected f rom Nyohini in the Tamale Metropolis. Foreign biomass and other materials were then removed f rom the f eed stocks. Groundnut and sheanut shells were kept in earthen pots and then transf erred into a Gallenkamp muf f le f urnace with internal dimensions of 18" x 8.5" x 7.5" High (1000 degrees centigrade, 220/40 volts, 16 amperes, H250). Gallenkamp muf f le f urnace was used to convert the f eed stocks into biochar under a limited oxygen condition. The slow pyrolysis of groundnut shell biochar was produced at 350 ± 5 °C f or 60 min and f ast pyrolysis at 700 ± 5 °C for 45 min in a muf f le f urnace. The slow pyrolysis of sheanut shell biochar was produced at 350 ± 5 °C f or 180 min and f ast pyrolysis at 700 ± 5 °C f or 90 min in a muf f le f urnace. The dif f erence in residence time of pyrolysis of groundnut and sheanut shells are due to their dif f erence in lignocellulosic biomass (Duwiejuah, 2017). Af ter producing, the biochars were lef t to cool, crushed to f ine powder, sieved through 0.2 mm and used f or the chemical analysis.

Biochar chemical analysis
The biochars were produced in July, 2019 and phytocertif ication was obtained f rom Plant Protection and Regulatory Service Division (PPRSD) in Tamale, this aided in the transportation of the biochars samples to University of Reading, Department of Geography and Environmental Science, United Kingdom. Biochars were crushed and grinded to a homogeneous f ine powder and dried up overnight at 105 °C preceding to ultimate analysis.
The pH was determined by weighing 10 g of biochar into a centrif uge tube (50 ml), and 25 ml ultra-pure water added using an automatic dispenser (BSI, 2005). The tube was caped and place on a shaker f or 15 minutes. A pH meter was used to determine the pH values of the biochar samples af ter the samples were kept f or 30 min (BSI, 2005).
Electrical conductivity was determined by weighing 10 g of air dried 2 mm sieved biochar into a centrif uge tube (50 ml), and 25 ml ultra-pure water added using an automatic dispenser (BSI, 2005). Samples of biochars were then kept f or 30 min prior to measuring the EC values using a pre-calibrated conductivity meter (BSI, 2005).
Biochars analysis of C, N and S were conducted in triplicates using an elemental analyser (Flash 2000, CE Elantech Inc, New Jersey, USA) (Analytical Methods Committee, 2006). Colourimetric method was used to determine P in the extract (Wang et al., 2012). Procedures involves extraction f or K, Ca and Mg availability with 1 M HCl (Camps-Arbestain et al., 2015).
Groundnut and sheanut shells biochars were milled, sieved, weighed into triangular glass bottles f or the determination of Na and Fe contents and total concentrations (Al, Cu, Mn, Zn, Cd, Co, Cr, Ni and Pb). Approximately, 0.5 g of biochar was accurately weighed on a f our-place balance using a plastic weighing boat (Alexander et al., 2006). The biochar was then caref ully transf erred into a Kjeldahl digestion tube (100 ml). Caref ully, nitric acid (10 ml concentrated) was added to each tube under a f ume cupboard and a glass bubble then placed on top of the tube (Alexander et al., 2006). They were then lef t in a f ume cupboard overnight. The tubes were placed in the digestion block the next day and cautiously heated to 60 C, lef t f or 3 hours and gradually increased to 110 C and digested f or 6 hours (Alexander et al., 2006). The tubes were removed f rom the block and were allowed to cool. The digestate was f iltered using prewashed Whatman 540 f ilter papers (12.5 cm diameter) into a 100 ml volumetric f lask, af ter which each volumetric f lask was topped up with ultra-pure water to the mark. Dilution with water was done by a f actor of two bef ore running them on inductively coupled plasma optical emission spectroscopy (Alexander et al., 2006). The apparent solutions were used to create the separate dilutions f or Na and Fe compositions and total concentrations determination in groundnut and sheanut shells biochars using dif f erent standard instruments. Atomic absorption spectroscopy (C10G-E050B Shimadz) was used to examine Na and Fe contents whereas inductively coupled plasma optical emission spectroscopy was used in the determination of Al, Cu, Mn, Zn, Cd, Co, Cr, Ni and Pb contents in the biochar. Samples were analysed f or each parameter in the University of Reading, Department of Geography and Environmental Science, United Kingdom Laboratory.

Data analysis
Pearson correlations matrix of chemical parameters of the biochars were determined. The PERI (potential ecological risk index) proposed by Hakanson (1980) was used to assess the potential ecological risk of potentially toxic elements in groundnut and sheanut shell biochars produced during slow and f ast pyrolysis. The ecological sensitivity, toxic level, and total concentration to potentially toxic elements were taken into consideration by this method (Kabala and Singh, 2001). The potential ecological risk index was calculated f ollowing the various steps below Equations (1, 2, and 3): where Cf a measure of the degree of pollution on potentially toxic element is the contamination f actor, Cm and Cn are the concentrations of each potentially toxic element in the mobile and stable f ractions, respectively, biological toxic f actor for each metal (5 f or Cu, 1 f or Zn, and 2 f or Cr) is Tr (Hakanson, 1980); potential ecological risk index of individual element is Er, and potential ecological risk index of the total pollution is PERI. The contamination f actor, potential ecological risk and potential ecological risk index values (Table  1) were used to evaluate the risk of metals in the groundnut and sheanut shells biochars. Er ≥ 320 -Very high contamination

RESULTS AND DISCUSSION
The chemical properties of biochars produced during pyrolysis temperature of 350 ± 5 °C and 700 ± 5 °C were shown in Table 2. The pH values of groundnut and sheanut shells biochars ranged f rom 9.42 to 10.23 and 662.33 to 3206.67 μS/cm f or EC.
The temperature of pyrolysis af f ected the chemical characteristics and quality of biochar. The pH of groundnut and sheanut shell biochars tend to be alkaline and upsurge with increasing pyrolysis temperature. Biochar that are alkaline in nature can promote adsorption of toxic metals and metal hydroxide precipitation f ormation and can also improve acidic soil (Ahmad et al., 2014b). Biochar has high immobilisation / removal abilities f or toxic metals in soil / water as a result of its excellent surf ace chemistry, for example, dif f erent f unctional groups, high surf ace area, high aromaticity and high alkalinity (O'Connor et al., 2018). A higher pH of biochars means they have more sites that are negatively charged f or binding toxic metal ions f rom deprotonation of hydroxyl f unctional groups, which can lead to higher capacities of adsorption (Mia et al., 2017). The pH determines the charges on the surf ace of biochar (positives or negatives). The pH of circumneutral being the predominant charges are negative and can be used to remove cationic metals f rom contaminated water (Wongrod et al., 2018). Similar studies, recorded pH values that ranged between 5 to 12 (Ahmad et al., 2014b). Biochar produced with temperature increasing f rom 350 °C to 600 °C resulted in increasing pH f rom 9.11 to 10.35 (Shan et al., 2020).
The EC f or sheanut shell biochar produced during slow pyrolysis was < 750 μS/cm implying the inadequacy of nutrient whilst groundnut shells biochar produced during slow pyrolysis was within the acceptable range of 750 to 2350 μS/cm. However, the groundnut and sheanut shells biochar produced during f ast pyrolysis were in a range that is sensitive f or tender plants, seedlings germination and can cause phytotoxicity. Understanding of the quantity of soluble salts biochar contain is paramount as high rates of its application to soil can adversely af f ect plants sensitive to salt  and phytotoxicity (Wilson et al., 2001). Biochar electrical conductivity knowledge was essential f or its applications in water, soil remediation and agriculture. Production conditions and f eed stock properties are the chief drivers of electrical conductivity of biochar (International Biochar Initiative, 2015). Electrical conductivity is dependent on the number of crystalline carbon structures, the porous structure and surf ace area of biochar (Jiang et al., 2013). It is related to water-soluble ions in the biochar (Rajkovich et al., 2012), and it af f ects communities of soil microbes, plant growth, and soil physical properties, by this means incidentally influencing nutrient cycling of soil . The total compositions of C and N, f or GB350, GB700, SB350 and SB700 ranged f rom 58.13% to 70.23% and 0.45% to 1.37%, respectively (Table 2). Maximum total carbon content was f ound in sheanut shell biochar produced during f ast pyrolysis and lowest was f ound in groundnut shell biochar produced during slow pyrolysis. The low temperature pyrolysis (350 ± 5 °C) did not permit concentration of carbon in the groundnut and sheanut shell biochars hence the reason for less total carbon contents. During f ast pyrolysis temperature, the groundnut and sheanut shell f eed stocks yielded higher total carbon content that showed there was depletion of hydrogen and oxygen during the process of pyrolysis. Since, carbon content increased with increase in temperature of pyrolysis which is signif icant (Uzun and Apaydin-Varo, 2018). Similarly, high total content of corn straw and soybean biochars were related to the O and H depletion during the process of pyrolysis (Zeng et al., 2018). Biochar carbon content must be greater than 50% of the dry mass as organic matter pyrolysed with lower than 50% carbon content are categorised as PCM (Pyrogenic Carbonaceous Material) (European Biochar Certif icate, 2012). In pyrolytic biochar, the proportion of carbon ranges f rom 50% to above 95% primarily depend s on the f eed stock instead of temperature of pyrolysis (Lu et al., 2020). Similar studies f ound total content of 67.78% f or corn straw biochar and 69.17% for soybean straw biochar (Sarf araz et al., 2020), 64.50% to 75.30% f or corn husk biochars produced at 600 °C and 500 °C, respectively (Sanka et al., 2020) which are within the range of this present study. Recent studies that reported higher carbon content than this present study was Chen et al. (2020) and Khan et al. (2020).
Concentration of nitrogen is relatively very low in all the biochars which is attributable to the high temperature during the pyrolysis conditions. As, the organic material burning results in the loss of nitrogen as volatiles (NO2, N2O and NH3) f rom the f eed stock (Sarf araz et al., 2020). Similar study also reported low N content, 1.36% f or BC600 and 1.11% f or BC800 (Khan et al., 2020) perhaps also due to N loss f rom the f eed stock during pyrolysis at high temperature. The properties of biochar are in direct quantity to its unusual N content in the parent f eed stock. Typically, legumes have additional N content in plant tissues (Sarf araz et al., 2020). At large, high N content of biochar can provide soil nutrients and enhance crop productivity.
The heteroatoms composition of the GB350, GB700, SB350 and SB700 ranged f rom 247.91 to 962.23 mg/kg f or S and 903.72 to 1948.38 mg/kg f or P. The S concentration in GB350 and GB700 is relatively higher than SB350 and SB700. The f inding of this study contrast that of Cheah et al. (2014) that reported that the amount of S is negligible in biochar. Phosphorus concentration in GB350, GB700 and SB700 are relatively higher and SB350 was lower in P content. Phosphorus content during the slow pyrolysis process can be preserved in f eed stock. At higher temperatures, P is reserved in the biochar (Qambrania et al., 2017). Biochar produced during low temperatures have additional soluble P that at high temperature becomes insoluble (Zheng et al., 2013). Phosphorus in biochar is responsible for adsorption of toxic metal f rom aqueous solutions (Li et al., 2017).
The mineral composition of the GB350, GB700, SB350 and SB700 ranged f rom 12944.92 to 20873.30 mg/kg f or K, 192.24 to 410.72 mg/kg for Na, 3567.98 to 13451.83 mg/kg f or Ca and 1150.33 to 3414.34 mg/kg f or Mg (Table 2). Potassium concentration in GB700 and SB700 is f airly high which is f ar greater than GB350 and SB350. Sodium concentration in GB350 and GB700 are higher than SB350 and SB700. Calcium concentration in GB700 is relatively higher than GB350, SB350 and SB700 and showed signif icant dif f erence between the biochars. Magnesium concentrations in GB350 and GB700 are relatively higher than SB350 and SB700. Similar study by Song and Guo (2012) f ound Ca, K, Mg and P content in poultry manure biochars to have increased by 32%, 31%, 30%, and 34%, respectively, when temperature of pyrolysis increased f rom 300 °C to 600 ºC.
Generally, the mineral content of groundnut and sheanut shell biochars increases with increasing temperature of pyrolysis. Mineral comp osition (Ca, Mg, K and P) in biomass and biochar, is also responsible f or metal adsorption f rom aqueous solutions (Li et al., 2017). Biochars with higher compositions of minerals can provide extra opportunities f or toxic metals adsorption f rom water. Toxic metals are adsorbed onto the biochar via exchange mostly with Mg, K and Ca however with protons f rom hydroxyl and carboxyl groups. The minerals f rom f eed stock biomass are not burned, so the pyrolysis process acts as a pre-concentration step of minerals. The pyrolysis conditions and variability in f eed stock have a significant ef f ect on the f orm and content of minerals in biochar (Zhao et al., 2015). Hence, quantity of mineral in biochars can dif f er based on the original biomass composition and as a f unction of conditions of pyrolysis employed (Shen et al., 2019).
The elemental concentrations in mg/kg of the GB350, GB700, SB350 and SB700 ranged f rom 1171.93 to 4325.60 f or Al, 20.76 to 27.23 f or Cu, 3487.03 to 10995.65 f or Fe, 92.39 to 219.29 for Mn, 25.36 to 33.37 f or Zn, 0.96 to 2.09 f or Co and 5.51 to 14.70 f or Cr whilst Cd, Ni and Pb were below detection limits (Table 2). Aluminum concentration in GB700 is relatively higher than GB350, SB350 and SB700. Copper was f ound only in groundnut shell biochars and was below detection limits in sheanut shell biochars. Iron concentration in GB350 and GB700 is relatively high which is f ar higher than SB350 and SB700. Manganese concentration in GB350 and GB700 was higher than SB350 and SB700. Similar study, f ound high concentration of 102.89 mg/kg f or Mn and 85.07 mg/kg f or Cu in biochar produced at 500 °C (Zhao et al., 2017). Heteroatoms (f or example N, O, P and S) are f requently present, whilst inorganic minerals (f or example Ca, K, Mg, Na and Si) and some toxic elements (f or example Al, As, Pb and Cd) may also be f ound in small quantities (Freddo et al., 2012). With the exception of Na, K is the low valence metal ion which is more available than Ca, Al and Mg that are high valence metal ions in the groundnut and sheanut shell biochars. Some of the chemical parameters of the groundnut and sheanut shells biochars were correlated (Table  3). Zinc concentration in GB350 and GB700 were a bit higher than SB350 and SB700. Cobalt concentration in GB350, SB350 and GB700 were a bit higher than SB700. Chromium concentration in GB350, SB350 and GB700 were a bit higher than SB700. Similar studies by Buss et al. (2016) established that the percentage availability of Cr, Ni, Cu and Zn increased with increasing temperature of pyrolysis.
Potentially, the groundnut and sheanut shell biochars can be used in f ields as a soil amendment. They will improve the overall quality of soil. Biochar ef f ect on the growth of plant is mainly related to dif f erent f actors, f or instance, biochar dosage rate, type of biochar, mixing depth, nutrients availability, soil texture and plant species (O'Connor et al., 2018). The water holding capacity can be improved by addition of biochar into soil which helps in retention of water f or a prolong period which is due to the highly porous structure of biochar (Liang et al., 2006). In an irrigational situation, reducing the f requency and intensity of watering will reduce the cost. In acidic soil, biochar addition has led to an increase in pH of soil (Glaser et al., 2002). Biochar addition in soil leads to increased cation exchange capacity which in turn lessens the nutrients loss through leaching (Lehmann, 2007). Since the biochar possess high CEC given it the ability to hold the nutrients available in the soil. As a result, it increases the use ef f iciency of nutrients in the soil which could have been washed away because of precipitation. Besides, the potential f or groundnut and sheanut shell biochars to trap nutrients via CEC and can upsurge K content in the soil. Biochar increased the K availability in soils through the enhanced CEC (Gul and Whalen, 2016).
Nutrient contents (Ca, K, Mg and P) are diverse in groundnut and sheanut shells f eed stocks due to the pyrolysis conditions. These bio chars can upsurge in soil essential nutrients (such as N, P and K), which are conducive f or plant growth. Previous studies revealed that biochar can be used f or supplying high quantities of Ca, K and Mg available to plants (Xu et al., 2013). The availability of phosphorus in groundnut and sheanut shells biochars showed the biochars are P rich can be used as f ertilisers. Hence, the nutrient-rich groundnut and sheanut shell biochars can be applied in arable soils as f ertilisers. Application of biochar can imp rove content of nutrient, particularly of N. Biochar influence the available and total N in soil which is linked to ammonia volatilisation, organic N mineralisation and denitrification / nitrification (Gul and Whalen, 2016). It also increased ef ficiency of N utilisation by crops and reduced accumulation ef ficiency of N and then enhanced the N bioavailability in agricultural soils (Zheng et al., 2013).

Potential ecological risk index
Copper, Cr and Zn in the groundnut and sheanut shell biochars recorded contamination f actors values which were less than 1 (low contamination), potential ecological risk index value in the range of ≤ 40 and PERI below ≤ 150 ( Table 4). The contamination f actor of individual potential toxic elements measures the individual metals degree of pollution, and its value is indirectly proportional to its possible of leaching (Devi and Saroha, 2014). Copper, Cr and Zn all showed potential ecological risk index value below ≤ 40 (Table 4). The PERI measured the degree of superposition of several harmf ul potential toxic elements on the environment and organisms (Li et al., 2013). The potential toxic metals in the groundnut and sheanut shell biochars have values that suggested low contamination and less potential ecological risk making the biochars ecof riendly.

CONCLUSION
Some chemical characteristics were dependent on temperature of pyrolysis and feed stock types. The mineral composition of biochars increases with increasing temperature of pyrolysis and will provide extra opportunities f or toxic metals adsorption. The biochars can be used in f ields as a soil amendment to enhance the overall quality of soil due to the high presence of total elements concentrations. They biochars can upsurge essential nutrients (N, P and K), in soil, which are conducive f or growth of plant and can be used to release slowly f ertilisers due to their richness in phosphorus. The potential toxic metals in the groundnut and sheanut shells biochars have values that suggested low contamination and less potential ecological risk making the biochars ecof riendly. Groundnut and sheanut shells biochars are promising f eed stocks f or water and soil remediation. Further research on some parameters and deeper understanding of their interactions between method of biochar production and f eed stock is important to serve as guidelines f or charring conditions and selecting f eed stocks based to their specif ic environmental and soil requirements.

Conflict of Interest
Authors have no conf lict of interest to declare cadmium