Assessment of goethite modified biochar on the immobilization of cadmium and arsenic and uptake by Chinese cabbage in paddy soil

ABSTRACT The combined remediation of cadmium (Cd) and arsenic (As) in paddy soils faces many challenges due to their opposite behavior. A pot study was conducted to evaluate the impact of biochar (BC), goethite (G), and goethite modified biochar (GBC) at a 1% application rate on the Chinese cabbage growth, uptake and immobilization of Cd and As in co-contaminated paddy soil. GBC composites were prepared at two different initial mass ratios (1:1 and 2:1) of iron/biochar (Fe/BC). Goethite modified biochar composite at Fe/BC initial mass ratios of 1:1 and 2:1 were labeled as GBC1 and GBC2, respectively. The results showed that GBC amendments significantly improved Chinese cabbage growth, chlorophyll content, and gas exchange attributes compared to BC and G applied singly. Biochar decreased Cd uptake by Chinese cabbage shoots and roots while increased As uptake by shoots and roots. The GBC1 amendment reduced Cd and As uptake by Chinese cabbage shoots (47.27% and 35.20%) and roots (36.80% and 41.12%), respectively, and reduced Cd and As bioaccessibility by 40.09% and 39.35%, respectively, compared to unamended soil. GBC is generally simple to synthesize from crop residue and is an environmentally acceptable material when used for Cd and As remediation in co-contaminated paddy soil.


Introduction
Heavy metal and metalloid pollution of soils is a serious problem throughout the world (Awad et al. 2019), which will induce endless damage to the environment. According to the Bulletin of the National Soil Pollution Survey in 2014 of China, about 19.40% of agricultural soil sites were polluted by heavy metals and metalloids (Khan et al. 2016).
The high toxicity of cadmium (Cd) and arsenic (As) and their risk for human exposure via the food chain is quite concerning. Cadmium is most frequently found as a cation, but inorganic As is primarily found as oxyanions of arsenite (As III) and arsenate (As V) (Li et al. 2020). Cd and As are prevalent soil contaminants with opposed geochemical properties due to their different charges. Elevated amounts of Cd and As in China's paddy soils due to human activity present a risk to food and environmental safety (Carré et al. 2017). As a result, reducing Cd and As mobility simultaneously in paddy soils is crucial.
Biochar is a carbon-rich material produced by the pyrolysis process under a limited oxygen supply (He et al. 2018). Previous studies have confirmed that biochar is an essential material for the immobilization of cations such as Zn, Cd, Pb, Cu, etc. Palansooriya et al. 2020). The dominant mechanisms for cations stabilization in the soil after biochar addition are precipitation, cation exchange, and surface complexation pathways Ali et al. 2020). Nevertheless, previous investigations revealed that raw biochar has a limited capacity for adsorbing anionic As because most of the biochar has net negative surfaces (Beesley and Marmiroli 2011;Zheng et al. 2020). Some researchers have indicated that biochar could decrease bioavailable As in soils (Qiao et al. 2019) and As concentrations in plant tissues (Strawn et al. 2015); thus, the efficacy of biochar application on various Cd and As contaminated soils is still not fully cleared.
Biochar adsorption capability for various anions (As III and As V) has been improved using numerous techniques. The metal-impregnated biochar known as modified biochar has been created by soaking biochar in iron, manganese, and aluminum solutions (Xu et al. 2018). When iron oxides (hematite, goethite, ferrihydrite, and magnetite) were used to modify biochar, the capability of biochar to adsorb anions increased (Khan et al. 2020;Zhang et al. 2021). A large surface area with numerous functional groups and pores may facilitate the growth of iron nanoparticles on biochar surface without becoming aggregated and oxidized, which improves the composite efficiency (Khandelwal et al. 2020). Goethite (α-FeOOH) is a common soil mineral investigated because of its effectiveness as soil and wastewater amendment (Zhang et al. 2021) with reactivity, porosity, large surface area, and capacity for heavy metals adsorption (Guo et al. 2015). However, the α-FeOOH nanoparticles have a tendency to be aggregated, which limits their application. Therefore, biochar loaded with iron oxides can avoid goethite aggregation and improve adsorption effectiveness (Yang et al. 2018).
Goethite-treated biochar exhibited a greater ability for adsorption of heavy metals than original biochar. This modified biochar was effective in removing tylson (Guo et al. 2016), copper (Yang et al. 2018), Cd, and As from water and acidic soil (Irshad et al. 2020;Zhu et al. 2020a, b). To our knowledge, little research has been done to evaluate the influence of goethite modified biochar on the uptake of Cd and As in plants. In this regard, Irshad et al. (2020) confirmed that adding goethite modified biochar to Cd and As acidic co-contaminated paddy soil enhanced rice yield and physiology and reduced Cd and As levels in the plant tissues.
Chinese cabbage (Brassica chinensis L.) is a common leafy vegetable representing 19% of total leafy vegetable production (Liu et al. 2010). Moreover, edible parts of leafy vegetables are accountable for accumulating 70% of total Cd in humans because of their absorption capacity to Cd (Khan et al. 2016). Therefore, to protect the food supply, it is critical to lower the levels of Cd and As simultaneously in vegetables, especially Chinese cabbage. Previous studies have shown that biochar application decreases cadmium and/or arsenic uptake by Chinese cabbage plants (Khan et al. 2016;Awad et al. 2019;Kamran et al. 2019). In addition, it was also stated that using iron modified biochar led to a 45.4% and 44.5% decrease in As absorption in roots and shoots of Brassica campestris L., respectively . However, as far as we are aware, there is no data on the influence of goethite modified biochars on simultaneous Cd and As uptake by Chinese cabbage growing in Co-contaminated paddy soil. Additionally, their impact on Chinese cabbage growth yield and physiological parameters under different Fe/BC mass ratios has not been investigated.
According to the above considerations, a Chinese cabbage pot experiment was conducted to examine the effect of biochar (BC), goethite (G), and goethite modified biochar (GBC) on As and Cd remediation in co-contaminated paddy soil. The main objectives were (1) to evaluate the impact of amendments on Cd and As immobilization in paddy soil with the aid of the toxicity characteristic leaching procedure (TCLP) and bioaccessibility test and, (2) to explore the role of amendments in decreasing the Cd and As uptake by Chinese cabbage, and (3) to assess the impact of amendments on the growth response and the physiological parameters such as chlorophyll content and gaseous exchange parameters.

Preparation and characterization of amendments
Rice straw was obtained from the research area of Huazhong Agricultural University, Wuhan, China. Rice straw was dried and crushed (2 mm) by a cutting machine. The ground samples were pyrolyzed in a muffle furnace at 550°C for 3 hours; the biochar was then ground to pass a 0.15 mm sieve. The goethite modified biochar amendments (GBC) were prepared using a co-precipitation method, and Fe (NO 3 ) 3 ·9H 2 O was used as described by Zhu et al. (2020b). Briefly, the 2.5 g and 5 g of biochar were mixed in 100 mL of 5 mol L −1 KOH solution and thoroughly stirred. The mixture was then treated with 89.5 mL of 1 mol L −1 Fe(NO 3 ) 3 ·9H 2 O solution and stirred for 1 hour. Afterward, the mixture was put in a capped container and aged in the oven at 60°C for 72 hours to facilitate the precipitation process. Finally, GBC was obtained by filtration, followed by repeated washing with water and oven-drying at 50°C. The original mass ratio of Fe/BC in this study was 2:1 and 1:1. The preparation of raw goethite was the same as the above steps but without BC. Table 1 illustrates the chemical properties of BC and GBC.
The Brunauer Emmett Teller (BET) specific surface area (SSA), average pore size (APS), and total pore volume (TPV) of amendments were assessed at 77 K using a standard physical adsorption analyzer (Autosorb-1, Quanta chrome, USA). Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were used to identify BC, GBC1, and GBC2 amendments. The FTIR samples were prepared using potassium bromide (KBr), whereas 1 mg of each amendment was mixed with 100 mg of oven-dried KBr (at 105°C), and the mixture was carefully ground. An FTIR spectrometer (Bruker Vector 22, with OPUS 2.0 software) was used to obtain sample spectra at wavenumbers from 400 to 4000 cm −1 . The XRD analysis was performed on a powder X-ray diffractometer (D8 ADVANCE, Malvern Instruments Ltd, Britain) using the Cu-Kα as a radiation source, generated at 40 kV. The samples were scanned over a 2θ range of 5-85°. Additionally, the surface morphologies of the amendments were characterized using scanning electron microscopy (SEM).

Soil samples collection, characterization, and analysis
The soil was sampled from a paddy field in Yangxin County, Hubei province, China (N 29°48ʹ40˝; E 115°25ʹ53˝). The dried soil was crushed and sieved to determine its physicochemical characteristics. The soil texture was measured using the pipette method and found that the soil was silty clay according to USDA classification with 14.38% sand, 52.58% silt, and 33.04% clay. The pH of the soil was 7.67, which was detected in the 1:2.5 soil to water (w/v) ratio. Soil CEC was 20.33 cmol (+) kg −1 , which was determined using the NH 4 OAC (pH 7.0) according to the description by Ali et al. (2021). The soil samples were digested using aqua regia (HCl -HNO 3 -HClO 4 ), and total Cd and As were determined with inductively coupled plasma optical emission spectroscopy (Agilent 5110 ICP-OES). Total Cd and As contents were 3.25 mg kg −1 and 64.36 mg kg −1 , respectively. The soil organic matter was 19.7 g kg −1 as determined by the wet oxidation method (Lu 1999), and the total iron was 37.99 g kg −1 .

Pot experiment
The pot experiment was conducted in the greenhouse at Huazhong Agricultural University, China. Each circle plastic pot received 1.5 kg of co-contaminated soil, and the pot size was 14 cm in height ×18 cm in diameter. Five treatments were conducted with three replications in a completely randomized design (CRD). The treatments were as follows: biochar (BC); goethite (G); goethite modified biochar (Fe/BC 1:1) (GBC1), and goethite modified biochar (Fe/BC 2:1) (GBC2) respectively at 1% application rate, while the control soil without any amendment was labeled as CK. The pots were incubated for 15 days to gain equilibrium at 70% of the water holding capacity (WHC). Chinese cabbage seeds were sown in each pot and thinned one week later; three plants remained for 60 days after sowing. Urea, potassium sulfate, and superphosphate were applied to the pots in the form of 0.19 g N, 0.13 g K 2 O, and 0.11 g P 2 O 5 for each replicate, respectively. Distilled water was regularly added to each pot during the experimental period to maintain 70% of soil WHC at 25-28°C temperature, 70% air humidity, and 16 h light duration.

Chinese cabbage growth and physiological parameters
The chlorophyll content value and gas exchange parameters in Chinese cabbage leaves were determined one day before the harvest. The chlorophyll content value (SPAD) was measured using a chlorophyll content meter (SPAD-502, Konika Minolta Sensing Inc., Japan). Gas exchange attributes (photosynthetic rate (A), transpiration rate (E), stomatal conductance (Gs), and intracellular CO 2 concentration (Ci)) were estimated using a photosynthesis system infrared gas analyzer (IRGA). The gas exchange attributes were measured between 10:00 and 12:00 o'clock with the maximum intensity of daylight. After harvesting, Chinese cabbage plants were cut into shoots and roots. The fresh shoots and roots were weighed immediately after harvesting. The roots and shoots were then dried at 60°C for 4 days until achieving a consistent weight, then determining their dry weight. The dried shoots and roots were ground for further analysis of Cd and As concentrations.

Cd and As concentrations in Chinese cabbage tissues
For determining Cd and As concentrations in plant samples (shoots and roots), the ground plant samples were digested in a 3:1 (v/v) solution of HNO 3 and HClO 4 at 180-200°C until the solution turned clear. The solution was then diluted to a volume of 25 ml with ultra-pure water (UP), then the solution was filtered and kept at 4°C. Cd and As contents in each sample were determined using ICP-OES.

Cd and As mobility after Chinese cabbage harvesting
After the Chinese cabbage harvesting, the solubility and bioaccessibility of Cd and As were examined in air-dried and crushed soil samples. The solubility of Cd and As was measured by the toxicity characteristic leaching procedure test (TCLP) using a glacial acetic acid solution as described by Ali et al. (2020). In summary, 20 mL of 0.1 M of a glacial acetic acid solution was used to extract 1 g of dried sieved soil (0.15 mm) from each triplicate. The soil suspensions were shaken for 18 h. After that, the supernatant was centrifuged for 20 minutes at 3180 × g, filtered, and preserved at 4°C for Cd and As analysis. A bioaccessibility test was used to examine the human bioaccessibility of Cd and As (Li et al. 2014). In brief, 25 mL of 0.4 mol L −1 of glycine solution (pH = 1.5) was used to extract 0.5 g of soil. The suspension was shaken for one hour at 180 rpm at 37°C; then, the suspension was centrifuged at 3180 × g for 5 minutes, then filtered.

Statistical analysis
One-way analysis of variance (ANOVA) was performed for statistics at the significance level of 0.05 using IBM SPSS 25.0 to compare the mean of the various treatments. All figures were created using Origin Pro 2018.

Characterization of amendments
The Brunauer Emmett Teller (BET) specific surface area (SSA), total pore volume (TPV), and average pore size (APS) of BC, G, and GBC are shown in Table 2. The SSA and TPV of GBC1 were 9.11 and 7.72 times higher than BC, but its APS was 1.6 times lower. The increase in SSA and TPV of the GBC amendments compared with BC may be caused by the addition of KOH during GBC synthesis, which might activate BC to enhance the mesopores and micropores of biochar and this result was also approved by SEM image (Figure 1a), as the GBC amendments had smaller pore size compared to BC. In addition, it was found that the SSA of GBC2 was lower than GBC1; this might be due to the increased iron loading, which leads to the formation of goethite aggregated chains on the biochar surface. Figure 1a shows the SEM images that revealed the surface morphologies of the BC, GBC1, and GBC2. BC had a smooth structure with a porosity range of 1-3 µm. SEM of GBC1 revealed a rough surface comprising nanoparticles without aggregation ranging from 0.6 µm -150 nm, which were identified as goethite nanoparticles. It was observed that GBC1 had a higher pore number and a smaller pore size compared to other amendments. On the other hand, GBC2 revealed some aggregation of goethite particles on the surface; this might be the explanation for GBC2ʹs lower SSA when compared to GBC1. Figure 1b shows the FTIR spectra of BC, GBC1, and GBC2. All three materials showed a peak at ~3350 cm −1 generated mainly by -OH/Fe-OH groups, and the peak at 1500-1600 cm −1 was related to aromatic C=C or C=O groups. The -OH functional group in GBC amendments was also enhanced compared with BC, which can be associated with incorporating Fe(NO 3 ) 3 and aided in partially coating the biochar surface with FeOOH. The peaks of 470 and 803 cm −1 of BC were due to the deformation of Si-O-Si and Si-O, primarily found in rice straw biochar. The C-O-C stretching vibration band was credited with approximately 1080 cm −1 . GBC amendments showed new peaks at 590 cm −1 and 445 cm −1 attributed to the ferrite bond (Fe-O) bending vibration (Chen et al. 2011), and the peak around 995 cm −1 was related to C-O stretching.
The XRD patterns of BC, GBC1, and GBC2 are shown in Figure 1c. XRD patterns of BC showed diffraction peaks at 2θ angles of 28.35°, 40.45°, 50.2° and 66.2° related to graphite-C crystal planes (PDF No. 23-0064) (Zhu et al. 2020a), as a result of the carbon skeleton structure of BC. The GBC amendments showed other peaks at 17.79°, 19.25°, 21.34°, 23.75, 26.32°, 33.32°, 36.82°, and 53.36° correlated to goethite crystal planes (PDF No. 29-0713) (Wang et al. 2015;Zhu et al. 2020b). These peaks indicated the formation and coating of goethite particles on the biochar surface. Overall, FTIR and XRD showed that GBC amendments had provided active sites for metal (loid) complexation by a large number of Fe-O and hydroxyl functional groups and covering with a large amount of goethite.

Effect of amendments on plant growth and physiological parameters
The fresh shoots weight increased by 19.82%, 9.03%, 47.51%, and 34.49%, while the dry shoots weight increased by 18.46%, 8.57%, 41.76%, and 32.31%, by adding BC, G, GBC1, and GBC2, respectively compared to CK (Table 3). The fresh roots weight increased by 27.79%, 14.36%, 58.86%, and 40.48%, respectively, while the dry roots weight rose by 34.46%, 16.87%, 62.31%, and 46.24%, respectively, by BC, G, GBC1, and GBC2 when compared to CK (Table 3). GBC additions, particularly GBC1, caused the most significant increase for both shoot and root biomass among the other amendments. All amendments showed a positive impact on the gas exchange parameters. Photosynthetic rate (A), transpiration rate (E), stomatal conductance (Gs), and intracellular CO 2 (Ci) of Chinese cabbage that grew in amended soil are shown in Figure 2. The application of BC, G, GBC1, and GBC2 enhanced photosynthetic rate by 25.15%, 10.29%, 62.75%, and 41.67%, and transpiration rate by 22.51%, 11.15%, 69.28% and 37.19%, stomatal conductance was increased by 14.86%, 10.83%, 50.56% and 29.58%, and intracellular CO 2 concentration by 17.00%, 11.07%, 50.34%, and 26.99% respectively, compared to CK. BC, G, GBC1, and GBC2 amendments were able to increase the SPAD (chlorophyll content) by 17.97%, 8.22%, 39.87%, and 23.56%, respectively (Figure 3). Chlorophyll content increased according to the following order: GBC1> GBC2 > BC > G > CK. Overall, the greatest increases in chlorophyll contents, photosynthetic rate, transpiration rate, stomatal conductance, and intracellular CO 2 concentration were found with the application of GBC1 and GBC2 amendments. CK (without amendments addition), BC is biochar, G is goethite, GBC1 and GBC2 are goethite modified biochars at Fe/BC initial mass ratios of 1:1 and 2:1, respectively. Each data point represents the mean of three replications with standard error. Means in the same column shown by different letters were significantly different at P < 0.05.

Figure 2.
Effects of BC, G, and GBC on photosynthetic rate (a), transpiration rate (b), stomatal conductance(c), intercellular CO 2 concentration (d). CK (without amendments addition), BC is biochar, G is goethite, GBC1 and GBC2 are goethite modified biochars at Fe/BC initial mass ratios of 1:1 and 2:1, respectively. Bars indicate standard error of the means (n = 3), bars with the same letter do not differ significantly (p < 0.05).

Effect of amendments on Cd and As uptake by Chinese cabbage tissues
The Cd and As content in the shoots and roots of plants growing in GBC amended soils was lower than in other treatments (Figure 4). Cd content in the shoot was significantly reduced by 26.99%, 17.78%, 47.27%, and 38.92% by applying BC, G, GBC1, and GBC2, respectively (Figure 4a). Similarly, Cd concentration in the roots declined by 23.04%, 15.61%, 36.80%, and 26.54% for BC, G, GBC1, and GBC2, respectively, compared to CK (Figure 4c). Compared to CK, BC slightly increased As content in shoots from 1.09 mg kg −1 to 1.18 mg kg −1 and roots from 6.49 mg kg −1 to 6.81 mg kg −1 (Figure 4b and d). G, GBC1, and GBC2 amendments reduced As concentrations in shoots by 14.63%, 35.20%, and 18.66%, respectively, and in roots by 22.24%, 41.12% 32.09%, respectively compared to CK (Figure 4b and d).

Effect of amendments on Cd and As mobility after Chinese cabbage harvesting
Following harvesting the Chinese cabbage, the solubility and bioaccessibility of Cd and As were shown in Figure 5. In comparison to CK, all treatments significantly (p < 0.05) declined the Cd bioaccessibility in soil. BC, G, GBC1, and GBC2 amendments reduced Cd bioaccessibility by 19.24%, 12.95%, 40.09%, and 28.67%, respectively (Figure 5a). On the other hand, the bioaccessibility of As was significantly (p < 0.05) changed with different treatments (Figure 5b). Compared to CK, BC slightly increased As bioaccessibility from 6.73 to 7.05 mg kg −1 . In contrast, G, GBC1, and GBC2 reduced As bioaccessibility by 19.04%, 39.35%, and 29.87%, respectively, compared to CK.
Similarly, all amendments significantly reduced TCLP extractable-Cd, most notably in GBC1 and GBC2 by 35.24% and 30.27%, respectively, compared to CK (Figure 5c). After harvesting Chinese cabbage, the bioaccessibility and TCLP extractable-Cd were reduced in the following sequence: GBC1> GBC2> BC> G > CK. Except for the addition of BC, all other amendments reduced TCLP extractable-As (Figure 5d). GBC1 and GBC2 had the greatest reductions with 35.52% and 25.94%, respectively. As a result, the decreasing values of TCLP extractable-As were in the order of GBC1 > GBC2 > G > CK > BC after Chinese cabbage harvesting.

Plant growth and physiological parameters under amendments application
Growth and physiological parameters are key indicators that can aid evaluate the damage caused by heavy metals toxicity to plants (Ali et al. 2021). High levels of Cd and As have been observed to inhibit the growth of Chinese cabbage roots and shoots (Zheng et al. 2017). In the current study, compared to all of the amended units, the amount of Chinese cabbage biomass in the control soil was the smallest. Physiological changes and reduced nutrient absorption by metal-stressed plants may be responsible for decreasing biomass (both shoot and root) in control soil (Azhar et al. 2019). However, an improvement in the biomass yield for all amended soil was observed in the current study, probably due to a lower Cd and As bioavailability and soil nutrient provision (Sohail et al. 2019).
Previous researchers have found that adding biochar to soil contaminated with Cd and As improved crop biomass Irshad et al. 2020). Our results showed that rice strawderived biochar significantly increased the growth of Chinese cabbage, which was the same trend with Kamran et al. (2019); they found that rice straw-derived biochar enhanced the growth of pak Figure 4. Effects of BC, G, and GBC on Cd concentration in shoots (a), As concentration in shoots (b), Cd concentration in roots (c) and As concentration in roots (d). CK (without amendments addition), BC is biochar, G is goethite, GBC1 and GBC2 are goethite modified biochars at Fe/BC initial mass ratios of 1:1 and 2:1, respectively. Bars indicate standard error of the means (n = 3), bars with the same letter do not differ significantly (p < 0.05).
choi grown in Cd polluted soil. These increases may be because biochar contains numerous hydroxyl, carboxyl, and aromatic groups, which increase the ion exchange point and may affect the nutrient absorption by plants (Kamran et al. 2019). In addition, straw-derived biochars could be loaded with macro-nutrients like phosphorous (P), nitrogen (N), and base cations like Ca 2+ and Mg 2+ (Manyà 2012), and these nutrients could improve plant yield.
Our study agreed with Hartley and Lepp (2008); according to them, goethite application could effectively increase ryegrass biomass (Lolium perenne cv Elka) in As polluted soil. However, the application of BC increased the biomass more than the G application in the current study. In this regard, Li et al. (2018) observed that BC improved the growth of Brassica campestris L in Ascontaminated soil more than ZVI application. Additionally, Hartley and Lepp (2008) found that limetreated soil produced the highest biomass than the iron oxide treatments; they found that iron addition might decrease P supply, decreasing biomass compared to lime-treated soil.
The GBC amendments showed the highest reduction of Cd and As uptake by Chinese cabbage tissues compared with single G and BC, which might improve the biomass more than that in single G and BC. These findings correspond with the results by Irshad et al. (2020), who applied biochar and Figure 5. Effects of BC, G, and GBC on bioaccessibility of Cd (a), As (b) and TCLP-extractable Cd (c) and As (d). CK (without amendments addition), BC is biochar, G is goethite, GBC1 and GBC2 are goethite modified biochars at Fe/BC initial mass ratios of 1:1 and 2:1, respectively. Bars indicate standard error of the means (n = 3), bars with the same letter do not differ significantly (p < 0.05).
goethite modified biochar for Cd and As co-contaminated paddy soil; they found that goethite modified biochar amendments increased rice biomass more than biochar. Our results indicated that GBC1 was the potential amendment within the increase of biomass, which correlates to a metal uptake reduction by GBC1. The reasons for improving plant growth under GBC amendments may be related to the dual effect of iron and biochar as sources of nutrients; they can modify the physiological characteristics of the cultivated soil. In general, the increment in biomass with different amendments may be related to reducing heavy metals solubility and bioaccessibility due to their immobilization in soil. Our results found a significant negative correlation between Cd and As uptake by shoots and the weight of fresh shoots ( Figure S2).
Arsenic toxicity has been shown to inhibit photosynthesis and decrease chlorophyll concentration in Pteris ensiformis (Singh et al. 2006) and Boehmeria nivea L (Mubarak et al. 2016). Likewise, Cd toxicity impaired photosynthetic activities in wheat plants (López-Luna et al. 2016) and Brassica chinensis L. when grown on Cd-contaminated soils (Zhu et al. 2019). The findings of this study indicated different increments in chlorophyll content and gas exchange attributes (A, E, Gs, and Ci) in the Chinese cabbage cultivated in BC, G, and GBC amended soils.
This study suggested that BC significantly increased chlorophyll content and gas exchange (A, E, Gs, and Ci) attributes. Similar findings have been discussed by Kamran et al. (2019); according to them, biochar caused a significant increase in the gas exchange (A, E, Gs, and Ci) attributes in the pak choi plant. Our findings suggested that rice straw biochar amendment could minimize Cd uptake by plant tissues, improve soil properties (Ok et al. 2015), and thereby enhance the physiological parameters (chlorophyll content and gas exchange) of Chinese cabbage plants.
GBC amendments enhanced biomass and the photosynthesis process of Chinese cabbage against Cd and As stress. GBC amendments application greatly increased chlorophyll content and gas exchange (A, E, Gs, and Ci) attributes compared to the single addition of BC and G. In this regard, Liu et al. (2017) stated that incorporating iron elements into Cd-contaminated paddy soil improved gas exchange and the chlorophyll content of rice plants. Overall, Cd and As concentrations in shoots/ roots were negatively correlated with physiological parameters ( Figure S4). It can also be seen that Cd pollution might have a highly significant negative effect on the physiological parameters than As pollution.

Effect of amendments on Cd and As uptake by Chinese cabbage tissues
Cd and As biogeochemical behavior in paddy fields is typically contradictory (Qiao et al. 2019). Therefore, there is an immediate need for a simultaneous decrease of Cd and As uptake by plants. In the current study, the application of BC, G, and GBC showed different impacts on Cd and As uptake by Chinese cabbage shoots and roots. The use of BC decreased Cd concentration in Chinese cabbage shoots and roots. Similarly, many studies discovered that biochar could decrease Cd uptake in Chinese cabbage (Zheng et al. 2017;Bashir et al. 2018;Kamran et al. 2019). Biochar treatment may reduce metals uptake by plants because of metal dilution and immobilization (Park et al. 2011). The porous structure of the BC and the functional groups on its surface may be complex and adsorb Cd ions in soil solution, thereby reducing Cd adsorption by plant tissues (Bashir et al. 2018).
Although BC had a positive effect on decreasing Cd uptake by shoots and roots, it had a negative impact on As uptake by Chinese cabbage. The same results were mentioned by Zheng et al. (2017); they found that applying rice straw-derived biochar increased As amounts in Brassica chinensis shoots by 19% on average. In contrast to BC, the G amendment significantly reduced As uptake by both shoots and roots. Our findings agreed with Li et al. (2018), who used zero-valent iron (ZVI) and biochar for Brassica campestris L grown in As-polluted soil and discovered that ZVI could decrease As content in shoots and roots, whereas BC raised As concentration. Overall, the increased concentrations of As in plants using BC may be attributed to increased As mobility in the soil.
Moreover, GBC amendments reduced Cd and As uptake by Chinese cabbage shoots and roots more than single BC and G amendments. In this regard, Tang et al. (2020) found that low-dose (1.5%) applications of Fe-based biochar had the most significant impact on limiting Cd and As uptake under a wheat-rice rotation system. However, Yin et al. (2017) explained that Fe-based biochar effectively reduced As uptake by rice, but it may cause enhanced Cd uptake by rice. The different results may attribute to the different properties of Fe-based biochar or different soil properties during the growth of crops. The findings of this study are supported by Irshad et al. (2020); they concluded that GBC decreased Cd and As uptake in rice plant tissues. In alkaline conditions, available Cd and As could interact with GBC via surface complexation and co-precipitation (Manasse and Viti 2007), thereby minimizing plant uptake. Fe functional groups on the GBC surface may also be a reason for reducing the uptake by Chinese cabbage tissues. Additionally, the current study indicated that Fe/BC ratio played a vital role in lowering Cd and As uptake, GBC1 was more efficient than GBC2 in decreasing both Cd and As uptake by shoots nearly 1.21 and 1.89 times, respectively, and in roots by 1.39 and 1.28 times, respectively. The reduction of Cd and As solubility and bioaccessibility in the soil might explain the decrease in Cd and As uptake by Chinese cabbage. Our findings indicated a positive correlation between TCLP extractable Cd /As and their uptake by shoots and roots ( Figure  S3). In general, leafy vegetables accumulate excessive heavy metals compared to other food crops (Li et al. 2021); however, the current study suggests that GBC amendments greatly decrease both Cd and As uptake by Chinese cabbage tissues.

Effect of amendments on Cd and As mobility after Chinese cabbage harvesting
Solubility and bioaccessibility tests were used to evaluate Cd and As mobility after cabbage harvesting. The current study found that the application of BC, G, GBC1, and GBC2 greatly affected the solubility and bioaccessibility of both Cd and As in co-contaminated soil. Our data indicated that Cd and As bioaccessibility fell dramatically as the concentrations of Cd and As in TCLP-extracts decreased. A highly significant positive correlation between TCLP and bioaccessibility of Cd and As was found ( Figure S1). Cd solubility and bioaccessibility in soil were reduced according to the following order: GBC1 > GBC2 > BC> G > CK after Chinese cabbage harvesting. BC significantly reduced Cd solubility and bioaccessibility but enhanced the solubility and bioaccessibility of As. This result agreed with Zheng et al. (2017). According to them, rice straw-derived biochar application caused more significant decreases in Cd mobility than As mobility. Biochar application can enhance Cd immobilization in contaminated soils due to its wide functional groups and porous structure (Mohamed et al. 2015).
According to our findings, all amendments except BC dramatically reduced As solubility and bioaccessibility, while BC application slightly enhanced As solubility and bioaccessibility in the soil after Chinese cabbage harvesting. Previous studies reported similar results (Fan et al. 2020), in which biochar increased As bioaccessibility in two mining arsenic-contaminated soils. Arsenic is primarily found in anionic form; therefore, the carboxylic and phenolic functional groups at biochar particles surfaces may have a low reactivity for As (Irshad et al. 2020), so the negative charges on the biochar surface (functional groups) may cause electrostatic repulsion between biochar and arsenic, making arsenic more mobile in the soil. As well as, alkaline agents can efficiently immobilize Cd while increasing As solubility (Zheng et al. 2020).
Likewise, Qiao et al. (2019) found that biochar declined the bioavailable As in rhizosphere soil. The different effects of biochar on As immobilization may be attributable to the different soil characteristics and biochar composition. Overall, the negative effect of biochar on arsenic may rule out its use in As immobilization in Cd and As co-contaminated soils. Goethite decreased Cd and As mobility in the current study; similar outcomes were found by Hartley and Lepp (2008), who confirmed arsenic reduction by goethite. On the other hand, Suda and Makino (2016) reported that Fe oxides could insolubilize As in soils; however, using Fe/Mn oxides to insolubilize cationic elements was generally ineffective.
The current study confirmed that GBC at different Fe/BC ratios played an essential role in reducing Cd and As mobility compared to single BC and G amendments. GBC1 amendment was more apparent than GBC2 for decreasing Cd and As mobility. Our findings were corroborated by Khandelwal et al. (2020), who used nano zero-valent iron loaded biochar to remove anionic metal species from soft water. They found that aggregated chains of nano zero-valent iron (nZVI) over the biochar surface were caused by increased iron loading, affecting the immobilization efficiency compared to the ratio of Fe/BC (1:1). Generally, GBC amendments showed the highest reduction of heavy metals mobility compared with single G and BC. The modification of iron on biochar decreased arsenic mobility, as mentioned by previous studies (Wang et al. 2015;Fan et al. 2020). The possible mechanisms for Cd and As immobilization by GBC amendments can be concluded by; (1) The fact that GBC has a higher surface area than single BC and G could explain this reduction, as (Wang et al. 2022) found that a Larger specific surface area may be beneficial concerning the adsorption phenomenon for As(III) and Pb(II), (2) A large number of hydroxyl and newly generated Fe-O functional groups and goethite nanoparticles coating supplied active sites for metal (loid) complexation by GBC, and (3) A portion of Cd and As could be diffused into the micropores on GBC amendments because of its porous structure.

Conclusion
The effects of biochar, goethite, and goethite modified biochar on the growth, physiological parameters, and uptake of Cd and As by Chinese cabbage plant tissues were evaluated in this study. Results showed that GBC amendments enhanced Chinese cabbage growth, chlorophyll content, and gas exchange attributes more efficiently than single BC and G additions. Additionally, GBC amendments under different Fe/BC mass ratios decreased Cd and As uptake simultaneously by Chinese cabbage roots and shoots compared to the single addition of BC and G.
Similarly, the GBC amendments showed the most significant reduction in bioaccessibility and TCLP extractable-Cd and As in soil compared to all other amendments. The ratio of Fe/BC should be considered during the application of goethite modified biochar. Our study indicated that GBC1 might be the best amendment for decreasing Cd and As uptake and improving Chinese cabbage growth and physiological parameters compared to other amendments. It can be concluded that GBC amendments contain a large number of hydroxyl and newly generated Fe-O functional groups and goethite nanoparticles coating, which might make them a supporting material for lowering Cd and As uptake, toxicity to plants, and bioaccessibility in paddy soil.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was supported by the Joint Key Funds of the National Natural Science Foundation of China (The Joint Key Funds of the National Natural Science Foundation of China U21A20237) and Research Program of Hubei Geological Survey Institute HBDZ-2020-02-55).