The soil environment has been a global concern due to its complexity and significance. However, with the rapid urbanization and industrialization various contaminants [such as HMs, radioactive elements and organics, etc.] are being introduced into the soil system through directly discharged to contain more than the allowable standard contaminant concentrations (Wang et al. 2020; Rybak et al. 2018; Feng et al. 2018). HMs have ranked the first among all soil contaminant types due to their high toxicity, bioaccumulation, persistence and mobility in soils (He et al. 2020; Yang et al. 2020). These HMs (Pb, Cr, Cd, Hg, As, etc.) can lead to potential health risks to predators and humans through the accumulation of food chains. For instance, Pb is a commonly recognized carcinogen and led to neurotoxicity, stomach and lung lesions; Cr(VI) is a strong oxidant and also act as carcinogenic and teratogenic characteristics, and both of them have been listed as priority monitoring and control pollutants (Hu et al. 2020). Excess of Pb and Cr in soil is mainly derived from intensifying anthropogenic activities including mining, sewage irrigation, and pesticide abuse (He et al. 2020), which could lead to a deterioration in various functions and stability of soil systems (Duan et al. 2018).
In the last several decades, various remediation technologies, including phytoremediation, excavation, landfilling, electrokinetic remediation, soil washing and their blending, have been used to Pb and Cr contaminated soils for removing or reducing high toxic element amounts (Liu et al. 2018; Sarwar et al. 2017; Trellu et al. 2016). However, most of these technologies due to their long remediation cycle, high energy consumption, low efficiency and generating significant secondary environmental impacts (Gerhardt et al. 2009), which are unsuitable for the most area to decelerate their development. Accordingly, there is an urgent need for inexpensive, efficient and stable amendment materials.
To date, various materials [resin (Chen et al. 2020); activated carbon (Dong et al. 2016); peat (Lee et al. 2015)] are used to adsorb HMs. Unfortunately, the resin has a potential risk with secondary pollution, activated carbon is expensive and peat has a characteristically low surface. Biochar is a material with carbon-rich, porous and high aromaticity which is obtained from the pyrolysis of biomass at relatively low temperature (< 700℃) under the presence of limited oxygen (Yu et al. 2020; Chen et al. 2020). Biochar as an environmental sorbent has become one of the most attractive research hotspots due to having abundant raw materials, easy preparation and stable performance, etc. (Hung et al. 2020; Zhang et al. 2020; Xiao et al.2019). In addition, biochar can increase crop yield through improve soil fertility and remediate soil by immobilizing HMs (Azeem et al. 2020; Xi et al. 2020). Numerous researches have showed that biochar plays important role in decreasing the HMs (such as Cr, Cd, Pb, Cu and Zn, etc.) total and unstable concentration (Puga et al. 2015; Gao et al. 2020; Liu et al. 2018). Thus, Biochar has great advantages as a green environmental sorbent in remediating HMs contaminated soil.
Biochar can be prepared from a wide variety of raw materials such as municipal sludge, livestock manure, crop straw (Al-Wabel et al. 2018). In recent years, municipal sewage sludge has increased sharply with the increasing improvement of sewage treatment facilities (Zhou et al. 2020a), it is a by-product of the sewage treatment municipal and contains a variety of harmful substances (pathogens, refractory organics and toxic HMs, etc.), which is easy to cause secondary pollution (Zhou et al. 2020b). Pyrolysis of sewage sludge is promising since it enables to decrease the harmful substances and volume. Meanwhile, the solid carbonaceous residues after pyrolysis (SSB) may be used as the amendment of HMs in soil (Zhang et al. 2018). At present, Some researches have showed that the excellent effect of sludge biochar on the remediation of HMs contaminated soil, For example, Penido et al. (2019) observed a significant reduction of Cd, Pb, and Zn bioavailability in HMs contaminated soil from a Zn-mining area by applying SSB. Fang et al. (2016) used the SSB to remediate soils that had been contaminated with cationic Pb(II), etc. and anionic Cr(VI), etc. respectively. However, there are still concerns regarding potential soil secondary contamination with toxic HMs due to SSB were thoroughly mixed with soil and require further investigations to the assessment of long-term risks (Fang et al. 2016; Figueiredo et al. 2019). Thereby, it is requisite to obtain a piece of equipment which is able to separate biochar from the soil when remediation ended.
In this study, biochar derived from sewage sludge and was placed in polymethyl methacrylate (PMMA) tubule and as an amendment inserted HMs contaminated soils. Herein, Pb and Cr were selected as representative of toxic HMs. The objectives of this study are to 1) investigate the effect of SSB on the fractions of Pb and Cr in soil, to 2) determine the changes of the total content of Pb and Cr in soil with the remediation distance from the SSB tubule and time. The research results will provide some theoretical references for HMs-contaminated soil remediation and passivation by biochar in future research and application.