Demolition waste for adsorption of metals: A step towards the circular economy.

The proposal of effluent treatment systems, based on circular economy principles, is a great challenge that leads to the reduction of waste from other activities, thus reducing the environmental and economic cost of the global process. In this work, buildings demolition waste is proposed to be used for metals removal from industrial effluents. To validate these hypotheses, tests were carried out on batch reactors using Copper, Nickel, and Zinc solutions, in concentrations between 8 and 16 mM. As results, removals greater than 90% were obtained. With these preliminary outcomes, it was decided to use equimolar multicomponent solutions with 8 and 16 mM of these metals in a column packed with demolition waste as adsorbent. From the breakthrough curves, it was observed that the adsorption occurs in the order Copper > Nickel > Zinc. The columns' saturated filler could be safely disposed of by incorporating it into conventional or special mortars and concrete. Preliminary studies on the leaching and resistance of mortars prepared with exhausted adsorbents are also promising. It is concluded that these materials emerge as an economic and sustainable alternative for metal contaminants removal.

Macrophyte biomass productivity for heavy metal adsorption.

The search for low cost adsorbents that have metal-binding capacities has intensified in the last decades. Some natural aquatic macrophytes have been studied as adsorbents to remove heavy metals. Macrophytes ease to propagate converts them into plague for many ecosystems while they are also considered by some activities as a residue; therefore its resignification implies positive environmental effects. Whereas these macrophytes can be obtained from water bodies where they develop naturally, controlled production in greenhouses may be more appropriate for its use as filling in fixed-bed reactors that must operate continuously throughout the year. This work focused on obtaining the macrophytes growth parameters in order to calculate the most proper greenhouse containers dimensions and their required cultivation periods to be employed in a determined fixed-bed reactor with a certain effluent flow to filter. These parameters include: the biomass yield, the area required to obtain certain dry weight of a given biosorbent, and the relative growth and propagation rates. Selected macrophytes species were Azolla pinnata, Salvinia molesta, Limnobium leavigatum, Lemna minor and Pistia stratiotes. The characterization of each biomass and the study of their performance as biosorbents were conducted for the removal of Cu (II), Pb (II) and Cr (VI) ions from aqueous solutions, always procuring the WHO guidelines for drinking water. Azolla pinnata resulted in the species with the highest percentage of dry weight (6.56%), and the lowest values of relative growth and propagation rates. This species was as well the most efficient in removal of Cu and Pb (96.7% and 99.4%, respectively), while Pistia stratiotes was better adsorbent for Cr with a removal of 58.8%. The possibility of metal recovery and macrophyte biomasses reuse was also proven. Given their natural abundance, elemental growth conditions and propagation rates, macrophytes represent a low cost alternative to the most efficient commercial adsorbents.

Simultaneous removal of chromate and phosphate using different operational combinations for their adsorption on dolomite and banana peel.

Chromate and phosphate are contaminant frequently present in industrial effluents such as tanneries. The objective of this work is to evaluate the efficiency of different operational combinations with dolomite and banana peel for the adsorption of phosphate and chromate in binary solutions. Both adsorbents are residuals from construction and food industries, respectively. Therefore, its use propitiates the reduction of treatment costs and it is an approach to the premises of the circular economy. In this work, the dolomite and banana peel adsorption efficiencies in simple and binary systems were studied. Equilibrium and kinetics tests were carried out in batch and in a fixed bed reactor. Dolomite was found to be selective for the adsorption of phosphate and banana peel for that of chromate. The mixture of adsorbents produced similar phosphate and chromate removal than each adsorbent individually. Therefore, the removals of both contaminants from binary solutions were tested using a fixed bed reactor filled with the mix of adsorbents and the breakthrough curves were analyzed. The obtained removals were 99% of phosphate and 70% of chromate. Finally, a brief discussion was held on the reuse and disposal of saturated adsorbents.

Dolomite used in phosphate water treatment: Desorption processes, recovery, reuse and final disposition.

Desorption is a method that contributes to two important aspects for the sustainability of the water treatments that involve adsorption processes: a) the regeneration of the adsorbent making it reusable in several efficient cycles and, b) the recovery of the adsorbate. In previous studies, it was shown that dolomite constituted an efficient adsorbent of phosphates from aqueous solutions. Once the adsorbent saturation is achieved, it becomes useless for further uses, generating waste, a new environmental problem if it could not be properly disposed. In this work, the main objectives were to study the phosphate desorption process for the recovery and reuse of dolomite and to evaluate the possibilities of a final disposal of exhausted dolomite for agricultural soil improvement and applying the desorbed phosphate as fertilizer. The most efficient agent for the desorption process was 1 M NH4Cl. Ultrasound evidenced a negative effect on desorption. The pseudo-second order model fitted better the experimental data and the equilibrium time was 30 min. Up to four efficient adsorption-desorption cycles were obtained. Phosphate bioavailability of exhausted dolomite was assessed with autochthonous microorganisms. The obtained extracts were used in Lactuca sativa growth experiments, establishing that they are not phytotoxic and otherwise, could promote the vegetal growth.

Eutrophication decrease: Phosphate adsorption processes in presence of nitrates.

Eutrophication causes aquatic environment degradation as well as serious problems for different purposes of water uses. Phosphorus and nitrogen, mainly as phosphate and nitrate respectively, are considered responsible for eutrophication degradation. The focus of this work was the study of adsorption processes for decreasing phosphate and nitrate concentrations in bi-component aqueous systems. Dolomite and hydroxyapatite were selected as low-cost adsorbents. Obtained results showed that both adsorbents have high capacity for phosphate adsorption which the presence of nitrate does not modify. Hydroxyapatite proved to be the most efficient adsorbent, however, it showed a low percentage of desorption and few possibilities of reuse. Dolomite, on the other hand, allows a desorption of the adsorbed material that favours its reuse.