Heavy metal contamination of soils is a pervasive environmental problem, impacting agricultural productivity, ecosystem health, and human well-being. Sources of heavy metal pollution include industrial discharges, mining activities, improper waste disposal, and the widespread use of fertilizers and pesticides. These heavy metals, such as lead (Pb), cadmium (Cd), chromium (Cr), and arsenic (As), are highly toxic and can persist in the environment, posing long-term risks due to their non-biodegradable nature and potential for bioaccumulation in the food chain. Heavy metals are metallic elements having relatively high density (5.0 g/cm⁻³) and might be toxic at lower concentrations. However, another definition of heavy metals states that it is a metallic chemical element having a relatively high atomic number and occurs naturally in soils and rocks via human activities which causes an increase in the metal concentration [1] [2] . Industrialization and human activities have led to an increase in the level of metal concentrations. Unlike organic contaminants, heavy metals are natural constituents of the earth’s crust and stay in the environment because they cannot be destroyed or degraded and tend to bioaccumulate [2] [3] . Anthropogenic activities such as smelting, mining, waste disposition, combustion of fossil fuel, and chemical and manufacturing activities have led to an increased level of heavy metals in the environment [4] [5] . Heavy metals have been viewed as hazardous chemicals that pollute the environment due to their toxic nature even at lower concentrations. Heavy metals that are most commonly found at contaminated sites include arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), Lead (Pb), which are non-essential, and copper (Cu), nickel (Ni), zinc (Zn), which are essential and may lead to health problems in humans [6] - [9] . These metals are highly toxic even in low quantities and the increased quantity of heavy metals due to emissions from industrial activities is of major concern as most of these metals are discharged into freshwater without sufficient treatment [10] [11] . Heavy metals have a long-term persistence in soil because they do not undergo chemical or biological degradation which makes soil the major sink for heavy metals release [12] [13] . Heavy metals stand out from other elements because of their high toxicity, bio-accumulative nature, and bio-magnification. The ability of heavy metals to bio-magnify and bio-accumulate could have long-term health effects on humans and their environments [14] - [16] .

Heavy metals in soil systems originate from many sources. However, the major sources have been identified as naturally occurring in the soil and human anthropogenic activities such as smelting of ores, waste incinerators, mining, coal combustion, fossil fuel combustion, industrial wastewater, sewage sludge or municipal compost, pesticides, and fertilizers [10] [17] . All these activities harm the quality of the soil which thereby affects soil properties like the chemical and physical elements in the soil [9] [18] [19] .

As stated by [18] [20] , due to industrialization, heavy metals and metalloids accumulate in the soil through emission from human activities and anthropogenic sources thereby affecting the soil quality conditions such as the soil pH, nutrients, moisture content, and humus content [21] . Several studies explained that heavy metal concentration in soil stays for a long time as it does not undergo microbial or chemical degradation but is bioavailable and changes are seen in their chemical form [3] [13] [22] . Biodegradation of organic contaminants hinders the presence of toxic metals in the soil. Heavy metal contamination poses a major problem to humans and the environment. The risks and hazards of heavy metals are great as they could contribute to global warming and may lead to serious health problems [23] [24] . A major entry pathway of heavy metals contamination into the human body is directly or indirectly through the food chain (soil-plant-human or soilplant-animal human), drinking of contaminated groundwater, reduction in food quality (safety and marketability) via phyto-toxicity, reduction in land usability for agricultural production causing food insecurity [2] [13] [25] - [27] . Soil contamination by heavy metals requires remediation as appropriate restoration methods of soil amendments are being developed [14] [28] . Furthermore, the increased shift from traditional techniques to greener and cheaper alternatives of soil remediation is frequently looked into; for instance, heavy metal transfer to groundwater and soil ecosystem can be reduced by situ metal stabilization [29] , the food chain toxicity and accumulation can be reduced by bioavailability, biochar can be used for immobilization of contaminated soil and water [5] [30] . Biochar, a carbon-rich material produced from the pyrolysis of organic matter under limited oxygen conditions, has emerged as a promising solution for soil remediation. Biochar’s porous structure, large surface area, and functional groups can enhance its capacity to adsorb and immobilize heavy metals, thereby reducing their bioavailability and toxicity. However, the effectiveness of biochar in retaining heavy metals depends significantly on the feedstock used and the conditions under which it is produced, particularly the pyrolysis temperature [31] - [34] . Softwood pellets, as a biochar feedstock, offer several advantages, including uniformity, high carbon content, and ease of handling. Additionally, the incorporation of grass chippings, an abundant and renewable organic resource, can potentially improve the nutrient profile and enhance the overall effectiveness of the biochar-soil amendment. This study focuses on investigating how the pyrolysis temperature of softwood pellet biochar, when used in combination with grass chippings, affects the retention of heavy metals in contaminated soils [34] - [37] . Despite the promising potential of biochar for soil remediation, several questions remain unanswered, how does pyrolysis temperature influence the physicochemical properties of softwood pellet biochar? What is the optimal pyrolysis temperature for maximizing heavy metal retention in contaminated soils? How do grass chippings interact with softwood pellet biochar to affect heavy metal immobilization? What are the long-term impacts of biochar and grass chippings on soil health and heavy metal dynamics? Addressing these questions is crucial for developing effective biochar-based soil remediation strategies and understanding the underlying mechanisms that govern heavy metal retention in soil. Numerous studies have explored using biochar for heavy metal immobilization in soils. Studies have shown that higher pyrolysis temperatures generally increase biochar’s surface area and pore volume, enhancing its adsorptive capacity for heavy metals. For instance, [33] [36] reported that biochars produced at higher temperatures effectively immobilized Pb and Cd in contaminated soils. [38] - [40] highlighted that biochars derived from different feedstocks, such as wood, crop residues, and manure, have varying capabilities in heavy metal retention. The type of feedstock used for biochar production significantly affects its properties and performance. [33] [37] demonstrated that the addition of organic matter, such as compost or manure, alongside biochar, could enhance nutrient availability and improve heavy metal immobilization in soils. This study differs from the previous studies as it aims to provide a detailed understanding of how pyrolysis temperature affects biochar properties and their efficacy in heavy metal retention. The novel aspect of combining softwood pellet biochar with grass chippings is explored to evaluate the potential synergistic effects on heavy metal immobilization and soil health. This study not only focuses on the immediate impacts of biochar and grass chippings on heavy metal retention but also considers the long-term implications for soil health, providing a holistic view of biochar application in contaminated soils.

This study aimed to identify the concentration of heavy metals in contaminated soil and how biochar can effectively retain these metals under flooding conditions. The XRF analysis was used to identify the presence of heavy metal concentration in the soil. The heavy metal concentration from highest to lowest for the soil samples is in order of As > Fe > Mn > Cr > Zn > Pb > Cu. This shows that arsenic had the highest concentration for the soil samples which might be as a result of heavy metal accumulation due to the presence of particle-size matter. The ICP-microwave digestion was used to analyze the amount of heavy metal present, however, this method is not a total extraction technique. The concentration of heavy metals from highest to lowest for the soil is Fe > Mn > Cu > Zn > Pb > As compared to the XRF which had the highest concentration. This means that the ICP technique is not the best option for analyzing arsenic due to interference that exists in the emission spectra.

FTIR analysis was conducted to find the relative contribution of the chemical functional group and show the sample’s aromatic nature. Also as a characterization tool, FTIR provides a clear picture of the effect of pyrolysis temperature on biochar chemical composition. A general observation showed that an increase in temperature resulted in a decrease in peak intensities. The most identifiable peak occurred in 1558 - 1568 cm⁻¹. Three peaks were seen between 800 - 1600 cm⁻¹ which can be associated with the C-H bonding. Most of the peaks observed in biochars having high pyrolysis temperatures can be found in the fingerprint region between 500 - 1800 cm⁻¹. The peak intensity around 1600 cm⁻¹ relates to C-C stretching vibration in aromatic compounds [44] , yet was less visible at 700˚C. The peak intensity related to aromatic nature increases at 550˚C while the peak becomes delicate at 700˚C. A study by [45] explained that peaks from 710 - 2500 cm⁻¹ are associated with carbonates. These peaks can be seen in biochar spectra produced at 550˚C temperature and are slightly visible at 700˚C. This means that a decrease in peak is observed at 700˚C as large peaks are usually not seen with FTIR spectra of highly aromatic chars.

This study showed that biochar pH ranged from (7 - 9) which is alkaline in nature. This can be a result of the biochar liming effect on acidic soils which shows that the soil surface is highly contaminated with heavy metals due to past industrial activities. Also, environmental hazards such as acid precipitation mobilize heavy metals and could increase the soil pH. Table 3 shows that EC indicates the amount of soluble ions in the soil. SWP700 had the highest EC compared to SWP550. Soil’s ability to exhibit both cations and anions is dependent on EC. This explains that at high temperature EC also increases and the statistical difference observed was (p < 0.05).

The SEM images represent biochar at different temperatures (550 and 700) Plate 2 , SEM is an efficient tool used to characterize the spatial distribution of biochar organic composition. The soft wood pellets showed several shapes with a size range from (5 - 500 μm) micro to macro pores. Biochar at 550 was glossy and 700 were flaky. Different magnifications were visible, larger pores were observed more in SWP700 as compared to SWP550, this can be due to the pyrolysis reaction rate at high temperatures. Different shapes in macropores and mesopores were observed as biochar porosity structure was determined, which explains the biochar’s large surface area.

The result reveals that DO with treatments having organic matter was low barely 1 hour after the start of the experiments compared to the control. This implies that DO consumption was fast as a result of the organic matter in the overlying water on which the bacteria feed. After the 1^st day, DO continued to decrease until the 15^th day while that of the controls dropped but started increasing on the 10^th day till the end of the experiments. DO of treatment 1 and treatment 2 (SWP550 & 700) having organic matter retained a low level all through the experiments. This is because more oxygen is consumed as the days go by, and the EC for the T₁, T₂, T₃ & T₄ (SWP550 and SWP700) with organic matter and without organic matter continued to increase till the end of the experiments. The highest concentration was seen on the 360^th hour. This showed how the ions dissolved in the overlying water. The increase was due to the release of soluble salts contained in the biochar used. Also, the increase can be a result of the rich ash contents contained in the biochars ( Figure 2 ).

Fe plays a major role in overlying water as it affects the exchange of trace metals. The grass chippings added to the treatments result in microbial decomposition. Studies revealed that the microbial reduction was a result of ferric iron (Fe³⁺) which serves as an electron receptor [3] [44] [46] . The Fe mobilization was obvious at the 120^th hour for treatment with organic matter. The set experimental conditions reveal that the amount of organic matter in the treatments affected anaerobic Fe reduction. Also, the concentration for T₃ (SWP700) was much higher due to less biochar than T₄ (SWP700) having more biochar. However, Fe concentration in SWP550 was lower as compared to SWP700 which shows that Fe released during the incubation period is affected by the presence of Fe in the soils. For the controls without grass chippings, a small amount of Fe was released. The reason might be due to the organic debris present in the samples. A significant difference was observed at the 120^th hour for both control, T₁ and T₂. Fe on the 120^th hour for T₃ showed significant differences compared to the 48^th hour and 360^th hour.

The equation shows Fe reduction during the water inundation experiment.

Fe(OH)₃ + 3H⁺ → Fe²⁺ + 3H₂O(1)

Manganese was released from the soil to the overlying water in all the treatments which can be attributed to anaerobic reduction of manganese oxide (MnO). The addition of organic matter resulted in the release of Mn just like Fe, the trend increased till the end of the incubation period. However, the release of Mn for T₂ (SWP550) was higher than for T₁ (SWP550) while the reverse is the case for SWP700. This can be attributed to the small amount of biochar present in the samples. A significant difference was observed for the controls at the 360^th hour compared to the beginning of the experiment. For T₁ and T₂ there was a big difference at the 120^th hour, 240^th hour, and 36^th hour which means that Mn increased and was released to the environment.

Arsenic release during the experiment could be observed and might be due to the reductive dissolution of iron. Ironbound with arsenic to give iron arsenate, while different trends were noticed during the experiments, the concentration of As was high at the 120^th hour for all the controls and treatments except for T₁ (SWP550) which was low as compared to T₃ & T₄ (SWP700). This suggests that high temperature also affected the amount of heavy metal released during the experiment. The amount of As in a particular time can be known and determined by the rate of immobilization. A significant difference was observed at the 360^th hour for both control and T₂. The As on the 24^th hour and 120^th hour for T₃ showed significant differences compared to the 48^th hour and 360^th hour.

Like iron and manganese, the release of zinc was due to the reductive dissolution of Fe and MnO. The concentration of Zn for the controls and treatments increased but the SWP700 was high till the 240^th hour. However, the concentration of SWP700 at 360^th hour was lower. This might be due to re-immobilization which occurred through hydrolysis. As described in the equation;

Zn²⁺ +H₂O → Zn(OH)₂ +2H⁺(2)

For the control, T₃ and T₄ no significant difference was observed as compared to the 120^th hour of T₁ and T₂ where the trend increased. The concentration of lead was low at the start but increased as the days went by. However, Pb release can be attributed to the partition of metals from different phases.

As a result of electrostatic attraction, chromium is reduced from Cr (VI) to Cr (III) to form chromium hydroxide. Biochar serves as an efficient reluctant due to the surface functional group and larger surface area. This mechanism explains why chromium was not detected for most of the samples, i.e., controls and treatments. Cd and Cu were barely detected throughout experiments. This might be a result of the small amount of both metals present in the soils. The findings from this research show that SWP550˚C is less effective compared to SWP700˚C. Also, a large amount of biochar at 700˚C is essential to effectively retain heavy metal in contaminated soil. The effect of biochar depends on feedstock type and pyrolysis temperature. However, Tang [47] used hardwood biochar to multi-contaminated elements (As, Cu, Cd, and Zn) where they observed AS and Cu mobilized compared to Cd and Zn which were immobilized in the soils, for biochar to be used as a soil amendment, a cost-benefit analysis should be carried out. However, the cost of biochar is dependent on many factors such as feedstock type, temperature, production scale, distribution, and handling.

This study shows that softwood pellet biochar serves as a tool for soil amendment and can remediate contaminated soil. Biochar serves as a cheaper and greener environmental sorbent. However, the type of biochar used in this study shows that SWP700 in large amounts is more efficient than SWP550. As regards contaminant removal, one type of biochar might not be appropriate. Studies show that biochar can increase the strength of metal toxicity in soils. It is important to investigate biochar efficacy in the stabilization of contaminants before using the biochar. The results of the research showed that SWP700 was more effective compared to SWP550. The reason behind the low efficiency can be attributed to certain factors such as pyrolysis temperature, feedstock type, contact time, and pH. Based on this further assessment needs to be done on the risks that may arise due to high levels of heavy metal in contaminated land and an investigation on the type of biochar properties and pyrolysis conditions should be carried out.

This research was supported by the Most Key Program of China (2023YFC3706802) and the Natural Science Foundation of China (No. 21377098).

Kyari Umar Donuma: Investigation, Data collection, Writing-original draft preparation. Limin Ma: Supervision, Validation, Funding, Project administration, Supervision, Resources, Validation, Funding, Writing-review & editing. Shu Xingquan, Yu Haiyan, Zhang Weiwei, Bu Chengcheng, Wang Yong-Sik, Luo Jiahong, Chen Guangyao, Yu Jinpeng, Zhang Ru, Han Yuchen, Zeng Haoyu, Matthew S. Wisseh, Amina Grema Mustafa, Mohammad Auwal Saidu, Mufidat Maman Khan: Writing-review and editing.

All authors have read, understood, and have complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors and are aware that with minor exceptions, no changes can be made to authorship once the paper is submitted.

The authors do not have permission to share data.