Iron and Magnesium Impregnation of Avocado Seed Biochar for Aqueous Phosphate Removal

Jihoon Kang, Jason Parsons, Sampath Gunukula and Dat T. Tran
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Jihoon Kang: School of Earth, Environmental and Marine Sciences, University of Texas Rio Grande Valley, Edinburg, TX 78539, USA
Jason Parsons: Department of Chemistry, University of Texas Rio Grande Valley, Brownsville, TX 78500, USA
Sampath Gunukula: Forest Bioproducts Research Institute, University of Maine, Orono, ME 04469, USA
Dat T. Tran: DEVCOM Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783, USA

Clean Technol., 2022, vol. 4, issue 3, 1-13

Abstract: There has been increasing interest in using biochar for nutrient removal from water, and its application for anionic nutrient removal such as in phosphate (PO 4 3− ) necessitates surface modifications of raw biochar. This study produced avocado seed biochar (AB), impregnated Fe- or Mg-(hydr)oxide onto biochar (post-pyrolysis), and tested their performance for aqueous phosphate removal. The Fe- or Mg-loaded biochar was prepared in either high (1:8 of biochar to metal salt in terms of mass ratio) or low (1:2) loading rates via the co-precipitation method. A total of 5 biochar materials (unmodified AB, AB + High Fe, AB + Low Fe, AB + High Mg, and AB + Low Mg) were characterized according to their selected physicochemical properties, and their phosphate adsorption performance was tested through pH effect and adsorption isotherm experiments. Fe-loaded AB contained Fe 3 O 4 , while Mg-loaded AB contained Mg(OH) 2 . The metal (hydr)oxide inclusion was higher in Fe-loaded AB. Mg-loaded AB showed a unique free O–H functional group, while Fe-loaded AB showed an increase in its specific surface area more than 10-times compared to unmodified AB (1.8 m 2 g −1 ). The effect of the initial pH on phosphate adsorption was not consistent between Fe-(anion adsorption envelope) vs. Mg-loaded AB. The phosphate adsorption capacity was higher with Fe-loaded AB in low concentration ranges (≤50 mg L −1 ), while Mg-loaded AB outperformed Fe-loaded AB in high concentration ranges (75–500 mg L −1 ). The phosphate adsorption isotherm by Fe-loaded AB fit well with the Langmuir model (R 2 = 0.91–0.96), indicating the adsorptive surfaces were relatively homogeneous. Mg-loaded biochar, however, fit much better with Freundlich model (R 2 = 0.94–0.96), indicating the presence of heterogenous adsorptive surfaces. No substantial benefit of high loading rates in metal impregnation was found for phosphate adsorption. The enhanced phosphate removal by Mg-loaded biochar in high concentration ranges highlights the important role of the chemical precipitation of phosphate associated with dissolved Mg 2+ .

Keywords: biochar; metal impregnation; phosphate adsorption; precipitation; pH effect (search for similar items in EconPapers)
JEL-codes: Q2 Q3 Q4 Q5 (search for similar items in EconPapers)
Date: 2022
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