Uncontrolled industrial discharge and agricultural runoffs often cause phosphate enrichment of surface water. This results in eutrophication which in turn threatens the aesthetic and ecological values of the aquatic system. On the other hand, global phosphorous sources are dwindling which necessitates its effective recovery and reuse. Hence it’s of high significance to develop effective phosphate recovery strategies from wastewater and sewage sludge 1, 2.
Biochar from waste commodities has become a promising eco-friendly adsorbent for phosphate remediation. Biochar offers suitable surface area, porosity, mechanical stability, and active sites which are desirable features for the effective adsorption of contaminants. Yet, pristine biochar has often poor adsorption efficiency and hence its properties can be further improved by surface treatments3, 4. Biochar modified with metals such as calcium, magnesium, aluminum, and iron, as well as rare earth metals or their combination, yielded better adsorption efficiency5–8. In addition, it’s equally important to consider feedstock types, as it influences the properties of the resulting biochar. In this regard, it’s essential to ponder the circular economy while selecting feedstock 3, 9. Hence, there are various waste commodities to which much value could be added through pyrolytic conversion into biochar. In this regard, coffee grounds being spent in large quantities could be a valuable source for biochar production and demonstrates a circular economy approach10–12.
Currently, most biochar-based adsorbent preparation often focuses on surface modification with metals. On top of surface modifications, the adsorption performance of biochar can be further improved by reducing its size to the nanoscale. Compared with its macro counterpart, nanobiochar (NBC) offers a higher specific surface area and increased adsorption sites. These features of NBC combined with the metal treatment could significantly improve the adsorption capacities of nanobiochar 13–15. NBC is commonly prepared by the ball milling method. Production of nanobiochar with the right particle size by ball milling requires optimization of parameters such as rotational speed, ball-to-power mass ratio, and milling time 16, 17. Although optimized ball milling could provide small-size biochar, it’s not easy to get such a facility in resource-limited areas. Hence, it’s essential to thrive for a simpler nanobiochar production approach.
In this study, NBC was produced from spent coffee grounds by simple acid digestion of the biochar in a hydrothermal autoclave reactor. This method is simple and nanoparticles of size down 2 nm were produced rapidly18. The resulting NBC was subsequently modified with magnesium to combine the higher specific surface area of the NBC and the excellent phosphate affinity of magnesium. Although modification of biochar with magnesium has been practiced 8, 19, a combination of its excellent phosphate affinity with nano-sized biochar is not been properly investigated. Batch mode adsorption/desorption studies revealed that the prepared Mg/NBC adsorbent has robust adsorption efficiency at a wider acidic pH range. Further, a circular economy is demonstrated by applying phosphate-laden nanobiochar as a phosphorus-release fertilizer which significantly enhanced the growth of garlic and beans in a pot experiment.