A circular economy-based valorization of agricultural residues to remove heavy metals from water: Adsorption and final disposal strategy

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This manuscript analyzes the valorization of agricultural wastes as heavy metal adsorbents and their eco-friendly final disposal as mineral admixtures to manufacture cement-based construction materials. The adsorption properties of nutshell, pistachio shell, and agave fiber for the removal of Pb2+ and Ni2+ from aqueous solution were analyzed under different operating scenarios (i.e., pH, temperature, waste dosage, CaCO3 concentration). Taguchi L9 experimental design was applied to identify the best removal conditions for both heavy metal ions. The maximum experimental adsorption capacities of Ni2+ were 14.8, 17.8, and 16.4 mg/g for pistachio shell, nutshell, and agave fiber, respectively, whereas the maximum Pb2+ adsorption capacities of these residues were 6.25, 16.25, and 103.5 mg/g. Adsorption studies showed that CaCO3 concentration of aqueous solutions affected the removal of these heavy metal ions. However, these agricultural residues showed a competitive removal performance under different water hardness conditions. On the other hand, the setting and compressive strength of cement paste samples containing these residues loaded with heavy metals (i.e., as mineral admixtures) were also investigated. The results indicated that the setting process of the cement paste samples was accelerated when the agave fiber was used in its natural form (without heavy metals), as well as when the pistachio shell and nutshell were saturated with heavy metals. These findings were associated with the agricultural residue structure, which allowed better adherence to the cement particles by hydrating the elongated form of the agave fiber and the globular form of the nutshell and pistachio shell. Finally, the compressive strength of cement-based specimens was evaluated at 7, 14, and 28 days after their preparation. The results showed that the cement paste samples incorporating nutshell had a higher compressive strength than those of the pistachio shell and the agave fiber because of the high lignin content in the nutshell. This circular economy approach for the valorization and final disposal of agricultural wastes is attractive to face their waste management problem, and the secondary pollution problem generated by the spent adsorbents used in water treatment.