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

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 (PO43−) 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 Fe3O4, 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 m2 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 (R2 = 0.91–0.96), indicating the adsorptive surfaces were relatively homogeneous. Mg-loaded biochar, however, fit much better with Freundlich model (R2 = 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 Mg2+.

Avocado Waste Biorefinery: Towards Sustainable Development

“…The porous structure of biochar also can retain water and nutrients, which can help reduce nutrient runoff and soil erosion. Studies have shown that biochar from avocado seeds impregnated with Mg-(hydr)oxide (postpyrolysis) demonstrated the benefits of metal impregnation regarding enhancing phosphate adsorption and its potential use for water treatment due to its performance for aqueous phosphate removal [99].…”

“…Composting seems to be the easier and cheaper alternative for avocado waste management. However, it can be a challenge due to some limitations, including (i) time and space requirements: composting requires time and space to allow the waste to decompose and transform into a usable product; (ii) high moisture content makes it difficult to compost it without the addition of dry organic materials such as straw or wood chips; (iii) improper composting process can result in a product with pathogen microorganisms such as Escherichia coli and Salmonella; and (iv) variability in composition, due to the composition of avocado waste can vary depending on factors such as the variety of avocado, the stage of ripeness, and the method of processing, which can affect the quality and consistency of the compost [70,99,101].…”

Hydrothermal Synthesis of Mg modified Nanobiochar from Spent Coffee Grounds for Effective Phosphate Removal: Kinetic and Isotherm Studies

“…Studies have indicated that such limitation of pristine biochar can be improved by surface modifications. [9][10] For instance, modification of biochar with magnesium (Mg/ biochar), [11][12] iron (FeO/biochar), [13] and aluminum (Al 2 O 3 / biochar) [13] resulted in improved adsorption efficiency. The adsorption performance of biochar can be further improved by reducing its size to the nanoscale.…”

Turning waste avocado stones and montmorillonite into magnetite-supported nanocomposites for the depollution of methylene blue: adsorbent reusability and performance optimization

“…For the current treatise, waste of avocado stones (AVSs) was selected as a biochar source (AVS-BC). The worldwide annual production of avocado exceeds 6.4 × 10 6 t. The stone (comprising the seed) constitutes 14-24% of the fruit, and the rest of the fruit is the peel and the pulp (García-Vargas et al 2020, Kang et al 2022. By and large, composting services do not accept the AVS which is hard to grind.…”