Effects of Calcination Temperature and Chemical Modication on the Adsorption of Cd and As(V) by Biochar Derived from Pteris Vittata

Phytoremediation can be applied successfully to solve the serious worldwide issue of arsenic (As) and cadmium (Cd) pollution. However, the treatment of biomass containing toxic elements after remediation is a challenge. In this study, we investigated the effective use of biomass resources by converting the As hyperaccumulator P. vittata into biochar to adsorb toxic elements. Plant biomass containing As was calcined at 600 °C, 800 °C, and 1200 °C and its surface structure and adsorption performances for As and Cd were evaluated. Calcination at 1200 °C increased the specic surface area of the biochar, but it did not signicantly affect its adsorption capacity for toxic elements. The calcined biochar had very high adsorption capacities of 90% and 95% for As and Cd, respectively, adsorbing 450 mg/g-biochar of both elements. The As adsorption rate was improved by FeCl3 treatment. However, the adsorption capacity for Cd was not signicantly affected by the NaOH treatment. In conclusion, it was found that after phytoremediation using P. vittata biomass, it can be effectively used as an environmental purication material by conversion to biochar. Furthermore, chemical modication with FeCl3 improves the biochar’s adsorption performance.


Introduction
Environmental pollution caused by anthropogenic and natural sources is a serious problem. Soil and water pollution caused by toxic elements, especially arsenic (As) and cadmium (Cd), can have grave consequences on human health. Arsenic is ubiquitous in the environment, including in the atmosphere, rocks, water, and organic matter 1,2 . Such health hazards caused by naturally occurring As have been reported in many Southeast Asian countries. Argos et al. estimated that tens of millions of people in Bangladesh and India suffer from health hazards caused by As ingested orally from drinking water and consuming food 3 . Arsenic ingestion can cause acute symptoms such as vomiting, diarrhea, muscle spasms, dysphagia, skin erosion, and death after coma 4 . As a chronic disease, in ammation of the mucous membranes of the eyes, nose, and throat is observed, and as the disease progresses, skin disorders such as black pigmentation and depigmentation keratosis appear, and the disease becomes more serious causing peripheral neuritis and cancer 4 .
Cd is released into the environment by the mining of zinc and other metals, and health hazards are often reported in river basins adjacent to these mines and smelters 5

. Many cases of health hazards caused by
Cd have been con rmed in Asia, and soil and water contamination have been reported in Thailand 6 and China 7 . Acute diseases caused by Cd in humans include nausea, vomiting, and abdominal pain, and chronic diseases, including tubular disorders, osteomalacia, and itai-itai disease 8 . Therefore, soil and water pollution caused by these toxic elements is a health hazard and an environmental problem that needs to be addressed promptly. To address these pollution problems, we focused on phytoremediation, an environmental remediation method using plants.
Phytoremediation is a method used for the removal of toxic substances from soil and groundwater using the material absorption capacities of plants 9 . In particular, research on methods using hyperaccumulator plants, which are plants that accumulate speci c elements at high concentrations 10 , is being actively conducted using Pteris vittata L. 11 , Thlaspi caerulescens 12 , and Alyssum lesbiacum 13 , which accumulate As, Cd, and nickel (Ni), respectively. The advantages of phytoremediation include lower cost than treatment using heavy machinery, lower environmental burden because it does not involve modi cation of the surrounding environment, and applicability to a wide range of lands 14 . However, while valuable metals such as Ni can be reused after accumulation 15 , the use of biomass with high concentrations of other toxic elements has not been studied, and incineration is the only option. Therefore, in this study, the conversion of biomass into biochar was conceived as a method to utilize the biomass after phytoremediation.
Biochar refers to the carbonized material produced by heating biomass at 250 °C or higher under oxygenfree or oxygen-limited conditions 16 . An important feature of biochar is that it possesses certain organic carbons, called fused aromatic ring structures, which are similar to charcoal. These structures are formed during pyrolysis and are key to the properties related to mineralization and adsorption. In addition, biochar has a large speci c surface area and high cation exchange capacity and is stable in the environment because of its resistance to degradation 17 . Because of these characteristics, biochar is an effective adsorbent for the restoration of environments contaminated by various substances, including heavy metals. It has been reported that biochar derived from Ginkgo biloba adsorbed up to 93.22 mg/L of Pb(II) and 22.58 mg/L of Cu(II) at an initial concentration of 100 mg/L 18 . Oil-palm empty fruit-bunch derived biochar and rice husk derived biochar adsorbed a maximum of 18.9 mg of As(III) and 19.3 mg of As(V) per unit weight, respectively, in a solution containing an initial concentration of 50 mg/L 19 .
Rapeseed straw-derived biochar adsorbed 32.74 mg/g of Cd(II) in 40 mL of CdCl(II) solution with an initial concentration of 1.25 g/L 20 . Furthermore, these studies have reported that the chemical modi cation of biochar can improve the adsorption of toxic elements. Thus, calcination temperature and post-calcination chemical treatment of biochar have been shown to improve heavy metal adsorption capacity, indicating that they are promising environmental puri cation materials. However, there are no reports on the use of hyperaccumulators as puri cation materials.
In this study, biochars of As-accumulated P. vittata biomass were prepared and their heavy metal adsorption properties for As and Cd were evaluated to effectively utilize them after their initial use for environmental remediation. P. vittata is known to accumulate As in its inorganic form, which is known to evaporate at temperatures below 200 °C 21 . Therefore, the As that was accumulated in the biomass during the biochar production process evaporates, and it will be relatively easy to collect and obtain a clean biochar. In addition, the biochars were chemically modi ed with FeCl 3 and NaOH to improve their heavy metal adsorption capacities.

Results And Discussion
Evaluation of surface structure and speci c surface area of biochar SEM images of untreated biochars of P. vittata and those chemically modi ed by FeCl 3 and NaOH that were calcined at 600 °C, 800 °C, and 1200 °C are shown in Figs. 1 to 3, respectively.
The leaf biochars at each calcination temperature ( Fig. 1) showed an increase in the number of particles on the surface of the grain as the ring temperature increased. On the other hand, there was no signi cant difference in the surface structure of the biomass as the calcination temperature was changed. Furthermore, in the case of chemical modi cations of leaf biochars, the FeCl 3 -treated sample ( Fig. 2) was found to have crystalline particles attached to the surface. In the NaOH-treated sample ( Fig.   3), some of the layered structures on the biochar surface were detached. In addition, cracks on the biochar surface were observed in all the FeCl 3 -and NaOH-treated samples.
Granular materials were identi ed on the surface of the non-treated biochar calcined at a temperature of 1200 °C, and they indicated that the functional groups on the surface of P. vittata leaves were decomposed and basic surface oxides were formed. Sasaki et al. reported that surface functional groups decompose and basic oxides are formed when carbon is heated in a vacuum or inert air stream at temperatures greater than 1000 °C 23 . Leaves contain more chloroplasts than stems, and thus have a large quantity of sugars, which explains the presence of granular material in the biochars of leaves calcined at 1200 °C.
The adhesion of crystalline particles was observed on the chemically modi ed biochar surfaces of samples in the case of FeCl 3 modi cation probably caused by the precipitation of Fe in the FeCl 3 on the surface, either directly or as complexes. Fe 3+ ions generated when FeCl 3 is dissolved in water are known to be reduced to Fe in the presence of activated carbon in the aqueous phase 24 , which explains the modi cation of Fe on the biochar surface. A porous structure caused by exfoliation of the surface structure was observed on the biochar surface treated with NaOH. Li et al. 20 reported that the speci c surface area of biochar was increased as a result of NaOH treatment. Therefore, it was considered that the surface of the biochar was eroded by the corrosive effect of NaOH resulting in the exfoliation of the surface structure and the generation of pores. In addition, the granular material identi ed in the leaf biochar calcined at 1200 °C disappeared after the NaOH treatment. This suggests that the granular material was composed of alkali-soluble substances. In addition, the cracks observed on the chemically modi ed biochar surface could be due to physical crushing by the agitator during the modi cation procedure.
The speci c surface areas of biochars at each calcination temperature are listed in Table 1. The speci c surface areas of the leaf biochar were 6.57 m 2 /g and 4.31 m 2 /g at 600 °C and 800 °C, respectively, with no signi cant difference. However, at 1200 °C, it was 34.54 m 2 /g, nearly 5 to 7 times higher. The results showed that the speci c surface area of the biochars increased with high-temperature calcination. In addition, the speci c surface areas of the Fe-modi ed leaf biochar were 160 m 2 /g, 114 m 2 /g, and 129 m 2 /g for calcination temperatures of 600 °C, 800 °C, and 1200 °C, respectively, displaying a maximum increase of almost ve times compared to that before modi cation. The speci c surface areas of the NaOH-modi ed biochar were 73.79 m 2 /g, 31.92 m 2 /g, and 65.38 m 2 /g at 600 °C, 800 °C and 1200 °C, respectively. In the case of the NaOH-modi ed biochar, the highest speci c surface area was observed at 600 °C. Therefore, the increase in the speci c surface area of the biochar was higher with FeCl 3 treatment than that of NaOH treatment. Considering the observation of the surface structure by SEM, it can be inferred that the speci c surface area of the FeCl 3 -treated sample increased because of the adhesion of Fe to the surface and the formation of pores by physical breaking of the biochar surface. In the case of the NaOH-modi ed biochars, exfoliation of the surface structure was also observed, which may be the reason for the increase in the speci c surface area. Surface cracks were observed in both FeCl 3 -and NaOH-treated biochars, suggesting that the adhesion of Fe to the surface is a more signi cant factor in increasing the speci c surface area.  20 . The surface areas of the FeCl 3 -modi ed biochars of this study using P. vittata exceeded the values reported in previous studies. In addition, the NaOH-modi ed biochars of this study had surface areas similar to those of the rapeseed straw biochar. Therefore, FeCl 3 modi cation of biochar has a greater effect on the increase in speci c surface area compared to previous studies.
Evaluation of As and Cd adsorption capacities of P. vittata biochar Figure 4 shows the adsorption isotherms of As(V) and Cd for the biochars calcined at 600 °C, 800 °C, and 1200 °C. Biochars tended to adsorb more Cd than As. The adsorption of Cd was particularly pronounced in the initial concentration range of 1-150 mg/L. At low initial concentrations (10 mg/L), calcination temperature of biochar did not have an effect on the adsorption of As. However, Cd adsorption increased in biochars that were calcined at high temperatures (800 °C and 1200 °C). At initial concentrations of above 100 mg/L, the adsorption capacities of the samples calcined at 600 °C and 800 °C were higher. The quantity of toxic elements adsorbed per unit weight of biochar at an initial concentration of 1000 mg/L was approximately 450 mg/g-biochar at any calcination temperature. The best adsorption values were 451.69 mg/g-biochar at 800 °C for As and 454.65 mg/g-biochar at 600 °C for Cd. Furthermore, Figs. 5 and 6 show the adsorption isotherms of As and Cd for the FeCl 3 -and NaOH-modi ed biochars, respectively. It was con rmed that Cd tended to be adsorbed more readily than As, even in chemically modi ed biochars. The best adsorption by biochar at the initial concentration of 1000 mg/L were 458.40 mg/g-biochar at 800 °C for FeCl 3 modi cation and 455.38 mg/g-biochar at 1200 °C for NaOH modi cation. Adsorption capacity increased by approximately 2% in the case of FeCl 3 modi cation, while it was not signi cant in the case of NaOH modi cation.
The adsorption rates of As and Cd on the biochars with and without chemical modi cation at each initial concentration are shown in Fig. 7. While the speci c surface areas of unmodi ed biochars were much larger at 1200 °C than at 600 °C and 800 °C, the adsorption rate of As(V) was only about 90% at any initial concentration. However, the adsorption rate of the FeCl 3 -modi ed biochars signi cantly improved at all initial concentrations at all calcination temperatures. In particular, it was shown that the adsorption rate in the low concentration range of 1 to 10 mg/L of initial concentration, which was not observed for unmodi ed biochars, was improved when biochars calcined at 800 °C and 1200 °C were used as feedstock. Although some improvement or decrease in adsorption performance was observed for biochars modi ed with NaOH, no signi cant change in the overall Cd adsorption properties was observed for biochars prepared at any calcination temperature as feedstock. The unmodi ed biochars showed a very high adsorption rate of about 97-98% under an initial Cd concentration of 1 to 10 mg/L, from which the adsorption rate decreased as the initial concentration increased, and the adsorption rate remained at approximately 90% for biochars at any calcination temperature at an initial concentration of 1000 mg/L. In the case of chemical modi cation with NaOH, the adsorption rate of the biochars calcined at 600 °C improved in the initial Cd concentration range of 1 to 10 mg/L, but the adsorption rate slightly decreased at higher concentrations compared to untreated biochars. In the case of biochars calcined at 800 °C and 1200 °C, the adsorption rate at low concentrations was lower than that of the unmodi ed samples, but it was slightly higher than that of unmodi ed samples for initial Cd concentrations of 100 to 250 mg/L.
Because the speci c surface area was evaluated using the monolayer adsorption model with the Brunauer-Emmett-Teller (BET) equation by nitrogen adsorption, it was not affected by the functional groups and the chemical state of the biochar surface. In contrast, the adsorptions of As(V) and Cd were not affected by the speci c surface area suggesting some sites on the biochar surface speci cally bind arsenate regardless of the calcination temperature. The adsorption of As(V) was not affected by the speci c surface area. On the other hand, Cd showed an extremely high adsorption capacity at low initial concentrations of 1 to 10 mg/L, while the adsorption rate decreased by 90% at an initial concentration of 1000 mg/L. This is because oxygen-containing functional groups, such as hydroxyl (-OH) and carbonyl (-COOH) groups, have high a nity and bind preferentially to cations such as Cd, but as the number of oxygen-containing functional groups decreases with increasing concentration, the ratio of binding by surface π-electrons increases. Therefore, this is a two-step adsorption mechanism.
The speci c surface area of the FeCl 3 -modi ed P. vittata biochar was larger than that of the unmodi ed biochar, and the As adsorption capacity improved. The speci c surface area of the biochar calcined at 600 °C was the largest among the three types, but the increase in As adsorption capacity by FeCl 3 modi cation was the highest for the biochar calcined at 1200 °C. However, the increase in the amount of As adsorbed by Fe-modi cation was the highest when the biochar calcined at 1200 °C was used as the raw material probably because the Fe 3+ ions were attracted to the π-electrons or oxygen-containing functional groups on the surface of this biochar than in the other two types 29 . The leaf biochar calcined at 1200 °C before modi cation has a larger speci c surface area than those calcined at 600 °C and 800°C , and in this case, the adsorption by π-electrons is stronger because the oxygen-containing functional groups on the surface are less due to the high-temperature calcination. Therefore, it is suggested that the π-electrons of the biochar calcined at 1200 °C attracted more Fe 3+ ions present in the aqueous solution, which increased the attachment of the reduced Fe particles to the biochar surface or they caused the formation of complexes with oxygen-containing functional groups resulting in an increase in As adsorption and speci c surface area. In addition, the reason for the improved arsenate adsorption at low initial concentrations of 1 to 10 mg/L in the case of FeCl 3 modi cation is that the biochars calcined at 800 °C and 1200 °C had a su cient amount of Fe that was adsorbed on to their surfaces, which may have resulted in the formation of a high-a nity complex between Fe and arsenate. In the biochar calcined at 600 °C, Fe was less modi ed than in the other two conditions, which may have caused the difference in the adsorption behavior after modi cation. On the other hand, it was con rmed that the adsorption performance tended to decrease at initial concentrations of above 100 mg/L compared to that at lower concentrations because the complex formation by Fe has high a nity, but the number of adsorption sites is limited, and hence the adsorption rate may have decreased due to the increase in arsenate present in the surrounding area.
Although the speci c surface area of P. vittata biochar modi ed with NaOH was larger than that before modi cation, the Cd adsorption capacity improved only at the initial Cd concentration of 100-250 mg/L probably because of the decrease in carboxyl groups (-COOH) associated with NaOH modi cation. It has been reported that the modi cation of biochar with NaOH results in the formation of lactones by dehydration condensation of the surface functional group COOH with the hydroxyl group (-OH) of NaOH and an increase in OH 20 . The P. vittata biochar calcined at 600 °C before modi cation had the highest amount of -COOH, and that calcined at 1200 °C was considered to have the strongest binding by πelectrons on the biochar surface. However, NaOH modi cation caused a decrease in -COOH, which is a surface functional group, and the presence of surface functional groups seems to have changed signi cantly.

Methods
Calcination of biochar P. vittata plants purchased from FUJITA Co. Tokyo, Japan. The raw material for biochar was P. vittata grown in a eld trial in Miyagi Prefecture. The concentration of As in P. vittata has been con rmed to be approximately 20 mg/kg 22 . All eld experiments are in compliance with local and national regulations. The harvested P. vittata was dried naturally, divided into leaves and stems. Then, only the leaves were packed in stainless steel containers and subjected to calcination in an electric furnace under N 2 atmosphere. During the calcination process in the electric furnace, the temperature was increased to below 150 °C for 3 h, from 150 to 600 °C at 100 °C/h, and for 600 °C and above at 200 °C/h. When the desired temperature was reached, it was maintained for 5 min to complete calcination. The biochar was calcined at nal calcination temperatures of 600 °C, 800 °C, and 1200 °C, and the obtained calcined biochars were used in the experiments.

Chemical modi cations with FeCl 3 and NaOH
The calcined biochars were chemically modi ed using FeCl 3 and NaOH. A quantity of 1.4 L of 1000 mg/L FeCl 3 solution was prepared and transferred to a 2 L beaker. The pH was then adjusted to 6 using 1 M HCl and NaOH. As a pretreatment, 7 g of leaf biochar was nely ground until more than 90% of it passed through a 1-mm sieve. The crushed biochar was added to the FeCl 3 solution and stirred with a magnetic stirrer for 24 h. After stirring, the solution containing the biochar was suction-ltered, and the resulting FeCl 3 -modi ed biochar was washed with Milli-Q water to remove the unreacted Fe. Then, it was dried in a dryer at 60 °C for 24 h. The above modi cation procedure was based on the method described by Sadegh-Zadeh and Seh-Bardan 19 .
The procedure for chemical modi cation with NaOH was based on the method described by Li et al. 20 . A quantity of 140 mL of 2 M NaOH solution was prepared and transferred to a 200 ml beaker. As a pretreatment, 7 g of biochar from leaves was nely ground until more than 90% of it passed through a 1-mm sieve. After grinding, the biochar was added to the NaOH solution and stirred vigorously for 12 h in a hot stirrer set at 100 °C with a magnetic stirrer. After stirring, the solution containing the biochar was ltered by suction. The resulting NaOH-modi ed biochar was washed three times with Milli-Q water and three times with 0.01 M NaHCO 3 . Then, it was dried in a dryer at 60 °C for 24 h.

Evaluation of physical properties of biochars
Structural observations using scanning electron microscopy (SEM) and measurements of speci c surface areas on a total of nine biochar samples including calcined and chemically modi ed with FeCl 3 and NaOH after calcination were performed Structural observation by SEM (JSM-6360, JEOL, Japan) was performed on the fractions of the samples that passed through a 1-mm sieve to evaluate the differences in structure due to calcination temperature and chemical modi cation.
A vacuum superheating pretreatment system (BELPREP-vac III, Microtrac Bell, Japan) and a speci c surface area measurement system (BELSORP-mini II, Microtrac Bell, Japan) were used to measure the speci c surface area. The sample biochars were crushed into small pieces using a mortar, set in the BELPREP-vac III, and heated at 130 °C for 3 h in vacuum to remove water and other substances adsorbed on the sample. After pretreatment, the sample was set in BELSORP-mini II, and the speci c surface area was measured using the nitrogen adsorption method. The BET method was used to analyze the speci c surface area of the obtained adsorption/desorption isotherms.

Evaluation of As and Cd adsorption capacities of biochars
Nine types of biochars were evaluated for their ability to adsorb As(V) or Cd. The samples that were chemically modi ed with FeCl 3 were evaluated only for As, and those chemically modi ed with NaOH were evaluated only for Cd.
The biochar samples were nely ground until more than 90% passed through a 1-mm sieve. After grinding, the samples were dried in a dryer at 115 °C for 3 h. After drying, the samples were placed in a desiccator and allowed to cool for 24 h. Then, three empty 50-mL centrifuge tubes were prepared for each sample. In each centrifuge tube, 50 mL of As(V) (Na 2 HAsO 4 · 7H 2 O) or Cd (CdCl 2 ) solution was prepared at concentrations of 1, 10, 100, 150, 250, 500, and 1000 mg/L. A quantity of 0.1 g of pretreated biochar was added to each centrifuge tube. The centrifuge tubes were sealed and shaken for 24 h at 100 rpm in a shaker at 25 °C with a lateral shaking width of 4-5 cm. After shaking, the supernatant was collected from the centrifuge tube and diluted to 10 mL with Milli-Q water by adding nitric acid to achieve a nal concentration of 5% (v/v). The test solution was then ltered through a 0.45 μm membrane lter and used as the sample for measurement. The As and Cd concentrations in the test solution were then quanti ed using high frequency inductively coupled plasma atomic emission spectrometry (iCAP6000, Thermo Fisher Scienti c, USA).