Figure 1. FE-SEM images and EDS elemental mapping results of BC and FeBC: (a) FE-SEM image of BC, (b,c) elemental mapping results of BC, (d) Fe-SEM image of FeBC, and (e,f) elemental mapping results of FeBC.

Figure 1. FE-SEM images and EDS elemental mapping results of BC and FeBC: (a) FE-SEM image of BC, (b,c) elemental mapping results of BC, (d) Fe-SEM image of FeBC, and (e,f) elemental mapping results of FeBC.

Figure 2. Results of adsorption isotherm experiments for each As species (As(V), DMA(V), DMMTA(V), and DMDTA(V)) for BC and FeBC. The maximal adsorption capacity (qm [mg/g]) and coefficient of determination (R2) of As(V) for BC were 1.28 and 0.2821, respectively. DMA(V), DMMTA(V), and DMDTA(V) were not adsorbed onto BC. For FeBC, the qm (mg/g) values of As(V), DMA(V), DMMTA(V), and DMDTA(V) were 6.32, 7.08, 0.43, and 0.28, respectively, whilst R2 values were 0.6837, 0.7920, 0.5075, and 0.7724, respectively.

Figure 2. Results of adsorption isotherm experiments for each As species (As(V), DMA(V), DMMTA(V), and DMDTA(V)) for BC and FeBC. The maximal adsorption capacity (qm [mg/g]) and coefficient of determination (R2) of As(V) for BC were 1.28 and 0.2821, respectively. DMA(V), DMMTA(V), and DMDTA(V) were not adsorbed onto BC. For FeBC, the qm (mg/g) values of As(V), DMA(V), DMMTA(V), and DMDTA(V) were 6.32, 7.08, 0.43, and 0.28, respectively, whilst R2 values were 0.6837, 0.7920, 0.5075, and 0.7724, respectively.

Figure 3. As reference compounds (As(V), DMA(V), DMMTA(V), and DMDTA(V)) and K-edge XANES spectra for two-line ferrihydrite, to which each As species was adsorbed. The dashed line represents the absorption energy of 11,873.8 eV.

Figure 3. As reference compounds (As(V), DMA(V), DMMTA(V), and DMDTA(V)) and K-edge XANES spectra for two-line ferrihydrite, to which each As species was adsorbed. The dashed line represents the absorption energy of 11,873.8 eV.

Figure 4. (a) LCF results of each As species adsorbed on two-line ferrihydrite (2LF) (open circles), against the As K-edge XANES spectra (solid lines). The R-factors for each analysis were 0.0105, 0.0014, 0.0027, and 0.0014 for the 2LF with adsorbed As(V), DMA(V), DMMTA(V), and DMDTA(V), respectively. (b) The stacked vertical bar indicates the ratio of As reference compounds (As(V), DMA(V), DMMTA(V), and DMDTA(V)) to compose the spectrum most similar to the spectrum of each sample, i.e., the proportion of As adsorbed to 2LF.

Figure 4. (a) LCF results of each As species adsorbed on two-line ferrihydrite (2LF) (open circles), against the As K-edge XANES spectra (solid lines). The R-factors for each analysis were 0.0105, 0.0014, 0.0027, and 0.0014 for the 2LF with adsorbed As(V), DMA(V), DMMTA(V), and DMDTA(V), respectively. (b) The stacked vertical bar indicates the ratio of As reference compounds (As(V), DMA(V), DMMTA(V), and DMDTA(V)) to compose the spectrum most similar to the spectrum of each sample, i.e., the proportion of As adsorbed to 2LF.

Figure 5. Proposed adsorption mechanisms of dimethylated arsenicals on FeBC.

Figure 5. Proposed adsorption mechanisms of dimethylated arsenicals on FeBC.

Table 1. Elemental mapping results (% surface Fe composition) for BC and FeBC, and total Fe concentration analyzed via aqua regia digestion.

Table 1. Elemental mapping results (% surface Fe composition) for BC and FeBC, and total Fe concentration analyzed via aqua regia digestion.

SampleSurface Fe Distribution (%) Determined Using EDS Elemental MappingFe Concentration (mg/kg) Determined Using Aqua Regia Digestion and AAS AnalysisBC0.2527FeBC17.647,155

Table 2. Changes in adsorption capacity for anionic contaminants before and after Fe modification for various biomass-based adsorbents.

Table 2. Changes in adsorption capacity for anionic contaminants before and after Fe modification for various biomass-based adsorbents.

Biomass UsedContaminantMaximum Adsorption Capacity (mg/g)ReferenceBiocharFe/Biochar CompositeHickory chipsAnionic dye (reactive red 120)2.9032.0Feng et al. [33]2.3979.4Corn stemAs(III)2.898.25 (Fe-Mn modified biochar)Lin et al. [35]Pine woodAs(V)0.2650.429Wang et al. [32]Corn strawAs(V)2.8614.77Fan et al. [34]Activated carbonAs(V)17.8620.24Yao et al. [36]Empty fruit bunchAs(V)5.515.2Samsuri et al. [37]As(III)18.931.4Rice huskAs(V)19.330.7As(III)7.116.9Activated carbonAs(V)0.004.663Tuna et al. [38]Rice strawAs(V)0.5525.923Wu et al. [39]

Table 3. Absorption energy (white-line energy position) (eV) of As species.

Table 3. Absorption energy (white-line energy position) (eV) of As species.

As SpeciesAbsorption Energy (eV)ReferenceAs(V)11,875.3Smith et al. [71]As(V)11,873.5Jing et al. [72]DMA(V)11,873As(V)11,875Maher et al. [73]DMA(V)11,873Monothioarsenate11,871.3Suess et al. [74]Dithioarsenate11,870.3Tetrathioarsenate11,869.8As(V)11,873Jeong et al. [69]DMA(V)11,872DMMTA(V)11,871DMDTA(V)11,870