Abstract
Biochar supported nanoscale zero-valent iron (NZVI/BC), prepared commonly by liquid reduction using sodium borohydride (NaBH4), exhibits better reduction performance for contaminants than bare NZVI. The better reducing ability was attributed to attachment of nanoscale zero-valent iron (NZVI) on biochar (BC) surface or into the interior pores of BC particles due to observations by scanning electron microscopy (SEM) and plan transmission electron microscopy (P-TEM) techniques in previous studies. In this study, cross-sectional TEM (C-TEM) technique was employed firstly to explore location of NZVI in NZVI/BC. It was observed that NZVI is isolated from BC particles, but not located on the surface or in the interior pores of BC particles. This observation was also supported by negligible adsorption and precipitation of Fe2+/Fe3+ and iron hydroxides on BC surface or into interior pores of BC particles respectively. Precipitation of Fe2+ and Fe3+, rather than adsorption, is responsible for the removal of Fe2+ and Fe3+ by BC. Moreover, precipitates of iron hydroxides cannot be reduced to NZVI by NaBH4. In addition to SEM or P-TEM, therefore, C-TEM is a potential technique to characterize the interior morphology of NZVI/BC for better understanding the improved reduction performance of contaminants by NZVI/BC than bare NZVI.
Highlights
Biochar supported nanoscale zero-valent iron (NZVI/BC), prepared commonly by liquid reduction using sodium borohydride (NaBH4), exhibits better reduction performance for contaminants than bare NZVI
Several possible ways were suggested to interpret the loading of NZVI on BC surface or in BC interior pores (i) Fe2+/ Fe3+ ions are adsorbed on surface or into interior pores of BC and be reduced to NZVI by N aBH415,20–22; (ii) Fe2+/Fe3+ ions are hydrolyzed into precipitates of iron hydroxide, and deposited on surface or into interior pores of BC particles for reduction to NZVI by NaBH423; (iii) Fe2+/Fe3+ ions are reduced to NZVI by NaBH4 in solutions and deposited on BC surface or into the interior pores of BC p articles[20,21]
There are a lot of chain-like clusters and roughly spherical particles dispersed on BC surface in scanning electron microscopy (SEM) images of NZVI/ BC (Fig. 3a,b)
Summary
Biochar supported nanoscale zero-valent iron (NZVI/BC), prepared commonly by liquid reduction using sodium borohydride (NaBH4), exhibits better reduction performance for contaminants than bare NZVI. The better reducing ability was attributed to attachment of nanoscale zero-valent iron (NZVI) on biochar (BC) surface or into the interior pores of BC particles due to observations by scanning electron microscopy (SEM) and plan transmission electron microscopy (P-TEM) techniques in previous studies. In previous s tudies[13,14,15,16], by scanning electron microscopy (SEM) and plan transmission electron microscopy (P-TEM) techniques, it was observed that NZVI particles were loaded on the surface or in the interior pores of BC particles, which is responsible for the improved reduction ability of N ZVI17–19. These experiments and the results could be helpful for exploring the underlying mechanisms to explain the better reduction performance of NZVI/BC than bare NZVI for contaminants
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