Heavy metal contamination has been a severe issue in many countries worldwide [1,2]. Human activities such as farming, mining, industry, and transportation are the primary sources that induce heavy metal pollution in the surrounding ecology [1,3]. Some heavy metals including zinc (Zn), copper (Cu), and manganese (Mn) are essential minerals to humans and plants at a minimal concentration and become toxic when their concentration is elevated [2,4]. At the same time, other HMs such as lead (Pb), cadmium (Cd), arsenic (As), and mercury (Hg) are toxic in trace amounts . Heavy metals (HMs) can cause widespread environmental pollution including soil, sediment, water, and plants . Consequently, this leads to the destruction of the ecology and induces a potential threat to human health via the food chain (soil-plant-human or soil-plant-animal-human) , especially to those who are dwelling near mining areas . Many studies have reported that mining activities are one of the most substantial sources of soil heavy metal pollution [6,7]. HMs present in the soil are stable chemicals, since they are non-degradable and persist in the environment; therefore, they have long-term effects on the soil and the ecosystem [1,8].
Many methods, which are based on physical, chemical, and biological processes, have been applied to remediate heavy metals in contaminated soil [2,4]. Chemical processes including chemical stabilization, electrochemical remediation, chemical soil washing, treatment with nanoparticles, and stabilization or solidification are very effective methods for remediating heavy metals in contaminated soil . Chemical stabilization, one of the most popular chemical processes, can decrease the mobility and bioavailability of heavy metals in the soil by adding specific amendments . The most commonly used modifications are various chemicals including limes [9,10,11], phosphate compounds [12,13,14,15], and organic compounds [16,17,18,19]. Nowadays, biochar, rich in carbon with large organic functional groups, is a ubiquitous amendment used in remediating heavy metals in contaminated soil [20,21,22,23]. It has been widely studied in remediating heavy metals in polluted soil since it has specific characteristics such as a somewhat high cation exchange capacity, porous structure, and large surface area . The ability of biochars to adsorb and stabilize heavy metals improves over time by forming organomineral microaggregates . Biochar is primarily produced from various biomass such as agricultural waste (rice straw, sugarcane bagasse)  and wood (willow wood, hardwood) , which are available and cost-effective materials .
Many previous studies have reported that corn cob-derived biochar has the potential to remediate contaminants in polluted soil [27,28,29]. Apatite ore (AP), rich in phosphorous, has been used to remediate heavy metals in soil in many previous studies [13,30,31,32,33]. The combination of biochar and apatite in remediating heavy metals in soil has also reported in some studies [34,35]. However, there is limited information about the effectiveness of the combination of various biochars with apatite in remediating heavy metals. This paper focuses on assisting in filling this information gap.
The present study aimed to study the impact of corn cob-derived biochar and the combination of biochar with apatite on the soil properties and speciation of heavy metals such as Pb and Zn, which dominate in the multi-contaminated farming soil. We hypothesized that biochar derived from corn cob and the mixture of biochar with apatite could transform heavy metals from labile fractions into stable fractions in contaminated soil. Therefore, this study was conducted to ascertain (1) the characteristics of corn cob-derived biochar produced at 400 °C (CB400) and 600 °C (CB600); (2) the alteration of soil properties after being treated with biochar and apatite; and (3) the effects of biochar and the blend of biochar and apatite (AP) to the chemical fractions of Pb and Zn, particularly the exchangeable fraction, for heavy metal remediation.