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Recovery of metals with high commercial value through adsorption by chars from rice wastes (Rice2Metal)
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Rice is the second most produced cereal in the world and Portugal is the major consumer and fifth larger producer in Europe. Its production generates significant amounts of wastes, namely rice husk (RH), rice straw (RS) and polyethylene (PE) from agricultural plastics. Currently, the destinations of these wastes are not the most environmental adequate, so different routes of valorisation are required. Having these materials interesting lower heating values (LHV), their valorisation in thermochemical processes, such as pyrolysis and gasification, became a possibility. These processes generate different products with energetic value (gasification generates mainly synthesis gas and pyrolysis mainly products are liquids, though some gases are also obtained). However, a solid fraction (char) is also produced in both thermochemical processes, which can be valorised as adsorbent materials.
Several industries have led to an increase in metal-contaminated wastewaters, which are directly or indirectly discharged into the environment, especially in underdevelopment and developing countries. Therefore, it is necessary to treat metal-contaminated wastewaters prior to their discharge into the environment. Chromium (Cr) and Tungsten (W) are two metallic elements that can be found in several industrial wastewaters. Additionally, the European Union (EU) published, in 2014, a list of 20 substances whose recovery is a priority for Europe. Chromium (Cr) and tungsten (W) were in this list due to their economic importance to the industry sector.
In this context, the main objective of this work was to evaluate the feasibility of using chars from the pyrolysis and gasification of RH, RS and PE, in the removal of Cr (as Cr(III)) and W (as WO42-) from aqueous solutions. In some situations, optimisation processes were necessary in order to improve the chars’ properties and efficiency in the adsorption/removal processes. For comparison purposes, a commercial activated carbon (CAC) was also used.
Regarding the several chars used on Cr(III) removal only two gasification chars (G4C and G5C) and one pyrolysis char (P1C) were selected to be used in the removal assays under batch conditions. Despite of the very low surface area of the chars, the gasification chars were used without any activation, due to their high mineral content, allowing removal by ion exchange. However, the pyrolysis char required further optimization to improve its adsorptive capacity. Different activations (physical and chemical) were applied to P1C char.
G4C char presented the best result among the gasification chars, achieving uptake capacities of 8.19 mg g-1 in the synthetic solution (3.93 mg g-1 for CAC) and 14.9 mg g-1 in the tannery industry wastewater (16.1 mg g-1 for CAC).
P1C physically activated (P1C+PA) presented the highest uptake capacities of all pyrolysis-derived activated carbons obtaining values of 9.23 mg g-1 in the synthetic solution (9.80 mg g-1 for CAC) and 12.4 mg g-1 in the industrial wastewater (16.1 mg g-1 for CAC).
G4C char was selected to be used in the column assays under dynamic conditions, but the performance of both G4C and CAC was lower than in the batch assays, obtaining uptake capacities of 1.60 and 2.14 mg g-1 in the synthetic solution and 3.25 and 7.83 mg g-1 in the industrial wastewater, respectively.
These results suggest that under batch conditions G4C and P1C+PA showed good properties to be alternative adsorbents in the removal of Cr(III) from liquid effluents, since their performance can be considered comparable to the commercial sample.
For the WO42 adsorption assays, six pyrolysis-derived activated carbons were produced.
The chemical activation with KOH originated the pyrolysis-derived activated carbon (P4C+KOH) with the best results on WO42 adsorption. The highest uptake capacities found for P4C+KOH were 854 mg g-1 in the synthetic solution, and 1561 mg g-1 in the industrial wastewater from a mining industry, while CAC’s values were significantly lower (113 and 572 mg g-1, respectively). P4C+KOH activated carbon clearly showed better properties than CAC on WO42- adsorption, obtaining uptake capacities almost 8 times higher in the synthetic solution and almost 3 times higher in the mining wastewater.
These results suggest that P4C+KOH seems to be a much more efficient alternative to CAC in the adsorption of WO42- from liquid effluents.
The main objective of the work was achieved as for both Cr and W removal it was possible to produce alternative adsorbents to the typical commercial activated carbon. Concerning Cr, the adsorbents produced obtained similar results to CAC, while for W the expectations were widely exceeded, as the produced adsorbents largely overcame CAC’s results.