Figure 1 Step by step preparation of AO-Fc adsorbent material

The chemical structure was confirmed by FTIR and XPS analysis. The FTIR spectra of Fc, CN-Fc, CN-Fc1, and AO-Fc were shown in Figure 2a. All spectra have similar shapes in the vibration band of carbonaceous material and a peak at 3500-3700 cm^-1 was observed due to the presence of OH groups^36,39. Afterward, Fc was modified with the aid of AIBN and DVB to yield CN-Fc. On comparing the FTIR spectrum of CN-Fc and CN-Fc1, the peaks in the spectra at 2242 cm^-1 were assigned to C≡N and high peak intensity witnessed the higher payload of nitrile groups were seen on CN-Fc rather than CN-Fc1⁴⁰. The post-synthetic functionalization further converted C≡N groups into C=N, C-N, and N-O but all of this was confirmed by respective peaks at 1638 cm^-1, 1380 cm^-1, and 928 cm^-1 indicating the presence of amidoxime groups on the surface of Fc⁴¹. The presence of amidoxime groups were further investigated by XPS analysis (Figure 2); in C 1s high-resolution spectra of AO-Fc (Figure S2a) showed two peaks at 284.45 eV and 285.5 eV assigned to C-N and C=N; in N 1s spectrum of AO-Fc (Figure S2b) showed two peaks at 400.05 eV and 398.40 eV assigned to N-H and C=N respectively³⁶, and the O 1s spectra of AO-Fc (Figure 2b) showed two peaks at 532.5 eV and 531.9 eV assigned to C(NH₂)-N-OH and O-H groups respectively⁴². The powder XRD patterns of Fc, CN-Fc and AO-Fc were shown in Figure 2c. The XRD patterns showed peaks around 16.2^o which can be indexed (101) plane and confirmed the amorphous nature of the material⁴³. Furthermore, a broad peak was seen at around 25^o and a small peak at nearly 43^o, which can be indexed (0002), (1010/1011) plane of graphite and are consistent with the presence of amorphous carbon⁴⁴. The contribution of amidoxime groups on the surface of AO-Fc was calculated by carrying out a thermogravimetric analysis. Compared with Fc, a significant weight loss (80 %) was observed in AO-Fc in the temperature range of 300 ^oC to 550 ^oC (Figure 2d). The higher weight loss in AO-Fc was due to a large number of organic groups being thermally decayed, which confirmed the weight contribution of amidoxime groups taken to 80 % of the weight loss of AO-Fc.

Figure 2 FTIR spectra (a), high-resolution XPS spectra of O 1s (b), powder XRD pattern (c), and TGA curves (d)

SEM morphology of the vertical and horizontal cross-section of Fc, CN-Fc, CN-Fc1, and AO-Fc were shown in Figure 3. The structure of Fc (Figure 3abc) was seen with two-dimensional rectangular two open-end vessels arranged in a proper array with a breadth of 28.6 μm (Figure S3a) and height of 18.3 μm (Figure S3b). SEM images of CN-Fc and CN-Fc1 were not changed obviously after chemical modification (Figure 3de). However, high magnified SEM images showed the rectangular walls were covered with small structures that confirmed the presence of chemical groups. In comparison to Fc, AO-Fc SEM images showed thick wall rectangular vessels (high magnified image) (Figure 3f) without any collapse or damage with a height and breadth of approximately 27.02 μm (Figure S3c) and 14.75 μm (Figure S3d). (Figure 3 j,h,i ) showed HAADF-STEM images of CN-Fc, CN-Fc1 and AO-Fc with the corresponding elemental mapping. These images clearly showed a high number of nano precipitates as illustrated in corresponding images (Figure 3 j,k,l,m,n,o) and their elemental percentage was reported in Table 1. These precipitates were mostly C, N, and O which are arranged in a rectangular homogenous pattern. The C and N elements of CN-Fc (Figure 3 j,k) and CN-Fc1 (Figure 3 l,m) were distributed in a rectangular fashion confirming the presence of C and N elements arranged in a homogenous pattern. (Figure 4 n,o) showed the presence of O, and N elements in rectangular patterns confirmed the successful conversion of nitrile groups into amidoxime groups.