Effect of calcium and iron-enriched biochar on arsenic and cadmium accumulation from soil to rice paddy tissues
Introduction
Rice is a staple cereal grain that accounts for over half of the global population (Jeon et al., 2011; Zhao and Wang, 2020). China produces 35% of global rice annually (Mohanty et al., 2010), and its rice production has more than tripled in the past few decades due to augmented yield (Jeon et al., 2011; Peng et al., 2009). However, rice is also a major source of dietary arsenic (As) and cadmium (Cd) intake in many Asian countries including China (Hu et al., 2013; Zhao and Wang, 2020). These metal(loids) are considered human carcinogens and are detrimental to human health (Zhao and Wang, 2020; Zwolak, 2020). As and Cd can accumulate in rice more easily than in other crops and then enter the food chain through animal or human consumption of the grains (Hu et al., 2013; Zhao and Wang, 2020). These metal(loids) are nonessential and perform no physiological or biochemical functions in plants (Williams et al., 2009). At high concentrations, they can inhibit root growth, diminish biomass and yield (Fattorini et al., 2017), and cause severe toxic effects (Williams et al., 2009) and plant death (Fattorini et al., 2017). The bioavailability of As and Cd in rice paddy fields is strongly influenced by the soil redox potential. However, the geochemical behavior and bioavailability of As and Cd respond differently to the paddy cultivation drying and flooding cycles (Honma et al., 2016; Pan et al., 2019; Qiao et al., 2018b). The bioaccumulation of As typically significantly increases under flooding conditions, whereas that of Cd decreases (Arao et al., 2009; Honma et al., 2016; Hu et al., 2013), which is in line with their opposite changes in solubility (solubility of As increases and that of Cd decreases) when the soil redox potential drops (Honma et al., 2016; Pan et al., 2019). Paddy soil Eh quickly declines after flooding because of O2 diminution, or because it is utilized in microbial activities that assist redox reactions. These reactions include reductions in sulfate, arsenate, manganese (Mn) oxides, iron (Fe) (oxy)hydroxides, nitrate, and methane production, and they are connected with the microbial oxidation of soil organic substances (Borch et al., 2010; Zhao and Wang, 2020). Cd solubility declines upon flooding in acidic paddy soils as Eh falls, which is largely a result of an increased pH, adsorption of Cd on Mn and Fe (oxy)hydroxides, and the formation and precipitation of cadmium sulfide (CdS) (Hussain et al., 2021; Kögel-Knabner et al., 2010; Li and Xu, 2017). However, a decrease in soil Eh enhances the reduction of As(V) to As(III) and desorption of As from Fe-(hydr)oxides, consequently increasing As solubility (Deuel and Swoboda, 1972). Hence, although flooding has been proposed as a measure to control Cd accumulation in rice, the simultaneous reduction of As and Cd uptake in paddy soils is more challenging than controlling a single pollutant (Irshad et al., 2020; Pan et al., 2019).
Various biological, physical, and chemical procedures have been proposed to diminish the impact of heavy metal pollution on ecosystems and agriculture (Pan et al., 2019); soil replacement, soil flushing, phytoremediation, and immobilization have been recently investigated as pollution-reduction techniques (Hussain et al., 2021; Komárek et al., 2013; Kumpiene et al., 2008; Marques et al., 2009). These recent techniques, except immobilization, are not particularly effective for large polluted areas and slightly contaminated farmland due to their propensity to cause secondary pollution, long remediation times, damage to soil microbial community structure and nutrients, and high costs (Cang et al., 2012; Vangronsveld et al., 2009; Zhang et al., 2013). In-situ immobilization using chemical agents is a widely investigated technique due to its high efficiency, low cost, and simplicity (Kumpiene et al., 2008; Qiao et al., 2018b; Wuana and Okieimen, 2011). Many chemical absorbents, such as clay minerals, Fe compounds, and biochar, have been suggested for heavy metal-contaminated soil remediation (Beesley et al., 2011; Hussain et al., 2021; Warren and Alloway, 2003). Biochar is a type of immobilizing material that has been studied recently; it has potential as an environmentally friendly, cost-effective, and widely accessible material for inactivating heavy metals (Gwenzi et al., 2017; Intani et al., 2019). The forms of geochemical binding and total concentration of potentially toxic elements or heavy metals exist in the soil are closely related with their potential toxicity, mobility, and bioavailability (Lu et al., 2017). These toxicity, mobility, and bioavailability of heavy metals in soils are indicative to their soluble and exchangeable forms, which could be altered via biochar amendment to the polluted soils (Pan et al., 2021; Yang et al., 2015). Biochar materials are typically effective in immobilizing cations such as Cd and Pb (Albert et al., 2021; Suksabye et al., 2016) due to a combination of pH adjustment and binding capacity. However, for anionic pollutants such as As, the immobilization effect of biochar is limited (Chen et al., 2016; Yin et al., 2017), and it can potentially increase As mobility by stimulating the activity of As-reducing microorganisms (Qiao et al., 2018a). Therefore, biochar materials may be a suitable option for immobilizing Cd in paddy fields, but they may not be the optimal choice for reducing the mobility and bioavailability of As in polluted soil (Pan et al., 2019).
To increase the effectiveness and applicability of biochar in immobilizing heavy metals, biochar composite absorbents have been prepared by adding Fe-, Mn-, calcium (Ca)-, and sulfur (S)-rich materials. These chemically modified biochar absorbents can reduce the bioavailability of both As and Cd in soils through adsorption and precipitation (Irshad et al., 2020; Islam et al., 2021; Lin et al., 2017). Fe-containing chemicals were selected because Fe-(hydr)oxides display a high affinity for oxyanions such as arsenate (Xu et al., 2017), and can improve the adsorbent functional groups. Application of Fe-enriched adsorbents can alter the chemical and physical properties of paddy soil and reduce the chemo-availability of soil As and Cd (Hussain et al., 2021; Kumpiene et al., 2008; Warren and Alloway, 2003). Moreover, Fe-enriched absorbents can improve the formation of root Fe-plaque, which can sequester significant quantities of metal(loids) (Irshad et al., 2020; Rajendran et al., 2019). Researchers have developed different Fe-enriched biochars (e.g., in the form of ferrous Fe, Fe oxide, and zero-valent Fe) that can improve the sorption capacity of biochar for As and Cd and reduce metal(loids) accumulation in rice grains in As and Cd co-contaminated paddy soils under various water management conditions (Irshad et al., 2020; Islam et al., 2021; Qiao et al., 2018b; Yin et al., 2017). The addition of Ca-enriched materials such as eggshells to the biochar can enhance the liming effect of the biochar, increasing the adsorption amount of cations in the soil matrix and biochar. Moreover, the adsorption of Ca ions on metal oxides can facilitate anion adsorption due to electrostatic synergy (Deng et al., 2019). Furthermore, sulfur-modified biochar and sulfate fertilization have attracted significant attention in recent years for enhancing nutrient uptake and paddy growth (Rajendran et al., 2019), improving root Fe-plaque formation (Rajendran et al., 2019), and reducing As (Zhang et al., 2015) and Cd (Rajendran et al., 2019) accumulations in rice grains.
Cost and environmental impact are two major considerations in selecting materials to produce immobilizing agents for heavy metals. Corncob is a widely available biomass residue that can be converted into biochar through pyrolysis for both soil improvement and energy production (Intani et al., 2019). Corncob biochar can be applied in agricultural production as a propagation substrate (Intani et al., 2019). In addition, corncob-based biochar has been used to immobilize metal(loids) in soils (Luo et al., 2020). Similarly, eggshell is an abundant waste by-product of the food industry and households. Eggshells exhibit a high Ca content, primarily due to their calcite (CaCO3) content (~90%); they have been used for environmental remediation and many other purposes (Ahmad et al., 2012; Guru and Dash, 2014). Ca is an essential plant macronutrient; it is a dominant regulator of the physiological and biochemical processes of plants due to its involvement in cell division, intracellular signaling transduction, cytoplasmic streaming, photosynthesis, and plant growth and development (Hirschi, 2004; Huang et al., 2017). The application of eggshells to acidic soil was observed to be particularly effective for improving the available state of Ca (Weiqi et al., 2018), although most soils are Ca-rich under ordinary conditions. Moreover, raw eggshells or eggshell-based materials have exhibited promising effects in immobilizing heavy metal ions (e.g., Cd, copper, lead, and zinc (Zn)) (Ahmad et al., 2012; Almaroai et al., 2014; Weiqi et al., 2018); hence, they can be utilized for soil remediation and wastewater treatment as a low-cost reagent.
To the best of our knowledge, in this study, Fe-enriched corncob biochar (FCB), Fe-enriched charred eggshell (FEB), and a combination of FCB and FEB (Fe-enriched corncob and eggshell biochar (FCEB)) were prepared for soil amendment for the first time. The effects of the amendments on As and Cd bioavailability and uptake in rice from a severely As and Cd co-contaminated paddy soil were analyzed and compared in this study. The biochar materials were also enriched in S when Fe was added in the form of Fe (II) sulfate (FeSO4). Our objective was to evaluate the added value of Fe-, S-, and Ca-enriched biochar when the materials were applied to paddy soils polluted with As and Cd. Hence, a pot experiment was conducted to explore the effects of FCB, FEB, and FCEB application on (i) paddy growth; (ii) rhizosphere pH and Eh, and porewater As and Cd; (iii) As and Cd sequestration by root Fe-plaque; and (iv) metal(loid) speciation in soil and contents in paddy rice. Moreover, possible mechanisms arbitrating the abridged As and Cd uptake by rice were predicted by assessing the contribution weights of the controlling factors for brown rice As (AsBR) and Cd (CdBR) accumulations.
Section snippets
Preparation and characterization of composite biochar materials
Corncobs and hen eggshells were used as the feedstocks for the biochar preparation. The corncobs were collected from a maize field in Wuqing District, Tianjin, China, and sun-dried for several days. The eggshells were collected from a canteen in Tianjin, China, and washed several times with deionized (DI) water. Next, the feedstocks were oven-dried separately at 70 °C for 24 h and then ground (20 mesh, 0.84 mm) before soaking independently in a 0.8 mol L−1 FeSO4·7H2O solution (solid to solution
Biochar characterization
The elemental compositions of the materials are listed in Table 1. The most critical components of FCB in descending order were carbon (C) (62.9%), oxygen (O) (23.1%), Fe (5.4%), and S (4.4%), whereas for FEB they were O (47.8%), Ca (28.7%), Fe (9.2%), C (7.8%), and S (4.2%). The specific surface areas were 205.8 m2 g−1, 25.3 m2 g−1, and 115.5 m2 g−1 for FCB, FEB, and FCEB, respectively.
The XRD (2θ = 10–90°) spectra of goethite, hematite, and magnetite were observed on both the FCB and FEB
Effective tillers, biomass production, and WUETDM
Soil amended with FCB, FEB, and FCEB, except 2% FEB, demonstrated enhanced plant growth, grain yield, and WUETDM (Fig. 2), which was possibly due to the reduced uptake of the metal(loids) (Figs. 3 and 4). In addition to the limiting effect, the biochar produced and used in this study (i.e., FCEB) can provide noticeable amounts of essential macronutrients (i.e., K, P, and S), secondary micronutrients (i.e., Ca and Mg), and trace micronutrients (i.e., Fe, Mn, and Zn) (Table 1) for improved growth
Conclusion
In this study, FCB, FEB, and FCEB were used to treat As and Cd co-contaminated paddy soil to explore their possible influences on the physiological responses of paddy plants, rhizosphere soil pH, Eh, porewater As and Cd contents, and metal(loid) accumulation in rice tissues. The results demonstrate that FCEB application to contaminated paddy soil enhances the growth and physiological development of the paddy plants. FCEB supplementation reduces metal(loid) accumulation in the paddy tissues and
CRediT authorship contribution statement
Md. Shafiqul Islam: Conceptualization, Methodology, Amendment material preparation, Pot experiment setup, Sampling, Investigation, Formal analysis, Writing original draft. Abdoul Salam Issiaka Abdoul Magid: Pot experiment setup, Sampling, Investigation, Writing, review and editing. Yali Chen: Conceptualization, Methodology, Multiple linear regression analysis, Writing, review and editing, Funding acquisition. Liping Weng: Conceptualization, Methodology, Multiple linear regression analysis,
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.
Acknowledgements
This research is supported by the National Key Research and Development Program of China (2017YFD0801003) and the Central Public-interest Scientific Institution Basal Research Fund (Y2020PT03).
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