The C content of the RS and RH biochar (~ 42%) was more than 2 times that of the PM biochar; while the content of N in PM biochar was 1.9 times the content in RS biochar, and 3.3 times the content in RH biochar (Table 1). In agreement with previous reports, the biochars prepared from manure contained lower C and higher N than the biochars prepared from plant residues (Xu et al. 2013; Zornoza et al. 2016). Such differences have previously been attributed to differences in the element composition of feedstocks (Zhao et al. 2013). In addition, the three biochars had different surface topography and pore structure (Fig. S1). The surface area, average pore volume and average pore size of PM biochar and RS biochar were all higher than those for RH biochar (Table 1), indicating PM biochar and RH biochar offer higher pore structures than RH biochar. This finding is in good agreement with previous literature highlighting that contrasting feedstock sources produced biochars with different physical and chemical properties (Sun et al. 2014; Hyväluoma et al. 2018).
The properties of biochar including surface area, distribution of surface functional groups, ash content and pH, play important roles during metal adsorption (Uchimiya et al. 2011; Jiang et al. 2016). As a consequence of differing physical and chemical properties, the sorption capacities of the three biochars varied (Fig. 1). Complexation of metals, through ion exchange interactions, with ionized surfaces and oxygen-containing functional groups (i.e. carboxyl (–COOH), hydroxyl (–OH), phenol (R–OH) groups) has been suggested as an important mechanism for metal sorption by biochar (Kołodyńska et al. 2017). In this study, RS biochar had highest surface area and highest content of surface functional groups (Table 1), available to prompt interactions with metal ions. Similarly, since the BET surface area and the content of surface functional groups of RH biochar were lowest among three biochars (Table 1), the sorption of Cd2+ ions onto RH biochar was correspondingly lower. In addition, the pH of three biochars also influenced the equilibrium pH and thus the adsorption of Cd ions (Fig. S4). The sorption of more Cd2+ ion with increasing pH is consistent with Zhang and Luo (2014) who also reported this relationship. In the batch experiments, the addition of RS biochar with highest pH lead to highest equilibrium pH among three biochars (Table S3); this, likely, underpinned the greatest sorption of Cd2+ ions onto RS biochar. Overall the RH equilibrium pH was lower than RS or PM, and Cd2+ ion sorption was also lower.
When amended to soil, biochars increased soil pH in all instances, but the increases were not significantly different (p > 0.05) among the three biochars (Fig. 2a). In addition, biochar had limited influenced on soil CEC (Fig. 2b). These outcomes are likely due to the low amendment levels (1.8 and 3.6 t/ha). Nonetheless, all three biochars significantly (p < 0.05) reduced extractable Cd concentrations (decreases followed the order: PM biochar > RS biochar > RH biochar) (Fig. 3). It is suggested, therefore, that changes in Cd availability were most likely linked to Cd ion interaction with biochar (rather than changes to the soil chemical environment). Reduced concentrations of available Cd were translated into observed reductions in Cd content in rape plants (Fig. 4 and Table S2). Importantly, the three types of biochar led to different outcomes for Cd-plant interactions. Treatment with RH biochar was relatively ineffective, while amendment with PM or RS biochar resulted in much more effective abatement of soil to plant transfer of Cd. Very little difference was observed where 1.8 and 3.6 t/ha application levels of the same biochar were compared. This observation suggesting, even at the lowest application rate (1.8 t/ha), that PM and RS biochars were effective ameliorants. The literature, in many cases, has reported metal-biochar-soil-plant interactions to result in large decreases in phytoaccumulation of metals into numerous crop types (Zhang et al. 2013; Puga et al. 2015; Xu et al. 2016; Younis et al. 2016; Zhang et al. 2016; Mohamed et al. 2017); while other cases this ameliorative influence has been reported to be minimal (Fellet et al. 2014; Hu et al. 2014; Lucchini et al. 2014; Kloss et al. 2015; Ree et al. 2015; Zhang et al. 2017). For instance, Zhang et al. (2017) reported that the amendment of biochar into heavily contaminated soils to have either no effect, or even to promote Cd accumulation into lettuce shoots. Wang et al. (2019) conducted a pot experiment with four types of biochar including wood biochar, rice straw biochar, Chinese walnut shell biochar and bamboo biochar. These authors reported none of the biochar amendments to influence Cu concentration of stems, leaves and roots of moso bamboo, while Cd concentrations decreased in all cases. The Zn accumulation in roots of moso bamboo was decreased in treatments with biochar, except bamboo biochar, while only wood biochar amendment reduced Zn concentration in plant stems and leaves. However, in the present study, RS and PM biochars (applied at low-levels: 1.8 and 3.6 t/ha) were established to be effective for the control of Cd phytoaccumulation, while RH biochar was observed to have only a limited effect.
While Cd sorption capacity of RS biochar was higher than that of PM biochar in the batch adsorption experiment, there were no significant difference between the decreased magnitude of Cd concentrations in rape plants where RS and PM treatments were compared (Fig. 4). These results highlight that the performance of these biochars in the batch adsorption experiments and in real soil systems were not consistent. This outcome is consistent with Uchimiya et al. (2010) who reported biochars produced from broiler litter manure at 350 °C (350BL) removed more Ni2+ and Cd2+ ions than biochars produced at 700 °C (700BL); but when applied (at 5% − 10 % (w/w)), to soil the 350BL treatment contained higher soluble metal concentrations than the 700BL treated soils. Although the 350BL had a higher adsorption capacity than the 700BL, its lower ability to increase soil pH underpinned the less effective immobilization of metals in soil by 350BL. Similarly, the pH increase, rather than primary Cd-biochar interaction, has also been proposed by other researchers (Houben et al. 2013; Rees et al. 2014). Thus, it is recommended that, before field scale deployment, biochar sorption capacity should be established in the presence of the soil it is intended to remediate.
In the present study, the pH of biochar amended soils were increased by 0.2–0.4 unit, while slight (although non-significant) changes in soil CEC were observed following biochar amendment. The minimal affect is most likely due to the low level of biochar applied (1.8 t/ha − 3.6 t/ha). Given that soil chemical properties (pH and CEC) were largely unchanged it is suggested that the primary interactions between Cd and the biochars were likely responsible for the outcomes observed.
The application of RS biochar or PM biochar at low level (1.8 t/ha) was effective to mitigate the transfer of Cd from soil into crop. With a doubling in application rates, the decrease of soil available Cd concentrations were increased (on average by approximately extra 10%), but the concentrations of Cd in rape showed limited change. It is highlighted that in the present research, the soil was not heavily contaminated. The soil Cd concentration (0.38 mg/kg) only just exceeding the regulatory limit of 0.30 mg/kg (MEP 2018). It is therefore emphasized that the low application rates of bichar were directed at a small excess of Cd in the soil system (this likely underpinned the successful outcomes observed) and that the lower application rate was sufficient to accommodate the excess of Cd. It follows that should soil Cd concentrations are much higher, such an outcome might not transpire and larger applications of biochar could be needed to accommodate a greater excess of Cd in the soil system. This said, Nie et al. (2018) reported the low level (1.5–3.0 t/ha) application of sugarcane bagasse biochar decreased the concentrations of Cd, Pb and Cu in pak choi by 62–76%, 17.3–49.1% and 15–38%, respectively. In contrast to the present study, the concentrations of Cd, Cu and Pb in this experimental were 1.4 mg/kg, 278 mg/kg and 348 mg/kg; Cd being more than 4 times the regulatory limit. Overall, the results of the current research support low-level application (i.e. 1.8 t/ha) of biochar to mitigate the transfer of Cd from soil to rape plants.
It is highlighted that many of the results reported in the literature relate to biochars that are produced in small quantities in the laboratory (Alburquerque et al. 2014; Zornoza et al. 2016; Bashir et al. 2018; Azhar et al. 2019). In contrast, the present study considered biochars produced using a larger pyrolysis system. The pilot scale pyrolysis system used to produce the biochars for this present research had an output capacity of 20 ton per year. Thus, assuming an application rate of 2 t/ha, such a unit could service 10 ha p.a. It is emphasised that this scale of production, and low application rates, represents a realistic approach to support the production of biochar in quantities that would allow for meaningful field scale application. Given the extent of diffuse pollution associated with a considerable proportion of China’s farmland (Lu et al. 2015a; Sun et al. 2019) and the predominance of small farms (< 0.6 ha) across much of China (particularly remote rural regions) (Zhang 2017), the collective research findings demonstrates the genuine reality of biochar-based remediation solutions to meaningfully contribute to mitigating some of the diffused contamination associated with tainted farmland.