The pH-sensitive sorption governed reduction of Cr(VI) by sludge derived biochar and the accelerating effect of organic acids
Graphical Abstract
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Introduction
Contamination of chromium (Cr) is widely distributed in the environment due to the discharges from industrial activities, including electroplating, metallurgy, leather tanning, dying, etc. (Cheng et al., 2014, Hausladen et al., 2018). Of the two major stable oxidation states, i.e., trivalent [Cr(III)] and hexavalent [Cr(VI)] states, Cr(VI) is soluble in water over a wide pH range and is highly toxic with carcinogenicity, mutagenicity and teratogenicity to living creatures, and thus is enlisted as a top-priority hazardous pollutant (Lyu et al., 2018, Zhou et al., 2016). Reducing Cr(VI) to Cr(III) is a favored treatment approach, as the transformed Cr(III) is much less toxic and, more importantly, is easier to be immobilized (Chen et al., 2015, Yang et al., 2018, Zhou et al., 2016).
Biochar, defined as the porous carbonaceous solid produced by the thermochemical conversion of biomass in an oxygen depleted atmosphere, has gained much attention in the area of environmental remediations due to its contributions to carbon sequestration and resource cycling, as well as its stable properties, high adsorptive capacities and multiple functions (Ahmad et al., 2014, Singh et al., 2010, Spokas et al., 2012). With regard to waste management and resource cycling, agricultural residues and municipal sludge are two of the most prevailing types of feedstocks for biochar production. The application of biochars derived from crop residues, sugar beet tailing, weeds and grass, etc., to Cr(VI) removal has been studied in recent decades (Choppala et al., 2012, Dong et al., 2011, Hsu et al., 2009). In addition to the traditionally emphasized sorption of Cr(VI), biochar could also effectively reduce Cr(VI) to Cr(III), and then the latter could be further immobilized on biochar through sorption (Liu et al., 2020, Yang et al., 2018, Zhou et al., 2016). These contributive reports demonstrated the general mechanisms involved in Cr(VI) removal by phytomass derived biochars. However, the effectiveness and dominant mechanisms of Cr(VI) removal by sludge derived biochar (SBC) have been discussed less frequently.
SBC has been proven to be safe and effective for heavy metal removal, nutrient recycling and soil remediation (Chen et al., 2020, Fang et al., 2016, Fei et al., 2019b, Khan et al., 2013, Yuan et al., 2016). It often features a high ash content and distinctive physiochemical properties (Zhao et al., 2013, Zhao et al., 2015), and thus the dominant mechanisms and key influencing factors of its environmental behaviors, including sorption, electron conduction and redox activity, may vary from those of phytomass based biochars (Vijayaraghavan, 2021). The binding and reduction of Cr(VI) by phytomass biochars are often believed to rely on organic functional groups, including hydroxyl and carboxyl groups (Mandal et al., 2017, Zhang et al., 2017, Zhang et al., 2018), while it is still unclear whether and how the enriched inorganic ash of SBC contributes to the removal of Cr(VI). The sorption of Cr(VI) or Cr(III) by SBC was studied (Agrafioti et al., 2014, Chen et al., 2015), while the reduction of Cr(VI) by SBC was often understated or attributed to the exogenous agents loaded on the SBC surface (Diao et al., 2018, Qiu et al., 2020, Zhang et al., 2013, Zhou et al., 2015). The specific process that occurs during the sorption and reduction of Cr(VI) by SBC must be further investigated in depth.
Therefore, laboratory experiments were conducted to determine the underlying mechanism in the present study. To investigate the all of the kinetic coupling processes of the sorption and reduction of Cr(VI) by SBC, batch tests that lasted for as long as 30 d were performed. Additionally, the effects of low molecular weight organic acids, i.e., malic acid, oxalic acid and citric acid, which are ubiquitously found in natural environments at low concentrations (Jiang et al., 2019, Tian et al., 2010) and are often selected as surrogates for natural organic matter, were also surveyed. The determining process of Cr(VI) removal by SBC was identified, which would of importance in understanding the chemical behavior of SBC, as well as for the development of SBC based applications.
Section snippets
Sludge-derived biochar and chemical reagents
Dehydrated sludge from municipal wastewater treatment plant was collected, dried and then pyrolyzed at 500 °C for 2 h. The obtained biochar, i.e., SBC, was ground to pass through a 60-mesh sieve (~250 µm) and employed for the present study. The elemental composition, ash content, pH and pHzpc of the SBC are listed in Table S1 in the Supplementary Information (SI). Chemical reagents, including K2Cr2O7, diphenylcarbohydrazide, acetone, malic acid, oxalic acid and citric acid were all of
Cr(VI) removal by sludge derived biochar
Cr(VI) removal by SBC under different pH conditions is summarized in Fig. 1. As shown, significant removal from the bulk solution was observed at pH = 2, as 14.2% and 26.0% of removal was achieved at 1 d and 30 d, respectively. While no such removal could be identified at pH = 4 or 6, despite the concentration fluctuation (<5%) due to the deviations in chemical determination. This is consistent with the literature that Cr(VI) removal is pH sensitive and that an acidic environment is favorable
Conclusions
The sorption of Cr(VI) on SBC was limited, but the efficient reduction to Cr(III) and effective immobilization of Cr(III) were observed. Sorption was the most pH sensitive process, and it governed the overall removal ratio. FTIR spectra identified the hydroxylic groups were highly involved in these processes. When malic acid, oxalic acid or citric acid was present, the reduction to Cr(III) was significantly enhanced, so the removal of Cr(VI) by SBC was accelerated at the early phase. However,
CRediT authorship contribution statement
Ying-heng Fei: Conceptualization, Methodology, Validation, Resources, Writing – original draft; Manzhi Li: Investigation, Validation; Zhuofeng Ye: Investigation, Validation; Jieyang Guan: Investigation; Zhenhong Huang: Investigation; Tangfu Xiao: Resources; Ping Zhang: Supervision, Data curation, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 41907119), the Natural Science Foundation of Guangdong Province (No. 2019A1515011617), the Guangdong Provincial Key Research Program of Universities (No. 2019KZDXM054), the Science and Technology Planning Project of Guangzhou (No. 202102010452), the General Research Project of Guangzhou University (No. YG2020013) and the Student Innovation Training Program of Guangzhou University (No. CX2019340). The technical
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