Smallmatek (Portugal)

The global scarcity of phosphorus and increasing concern over aquatic eutrophication are driving greater interest in phosphorus (P) recovery from water. Emerging materials, such as Layered Double Hydroxides (LDH) or their calcined forms, Layered Double Oxides (LDO), are gaining attention for their excellent sorption capacity. This study aims to evaluate the performance of three different materials, a calcined MgAl-LDO, ZnAlNO3, and its calcined product ZnAl-LDO for phosphate removal in batch conditions. The foremost strength of this article is that the materials under studied are manufactured on a large scale, thereby facilitating the scaling up for practical applications. The materials were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) before and after sorption. After calcination, MgAl-LDO reacquires its initial structure when in contact with an aqueous solution due to the “memory effect”, whereas ZnAl-LDO maintains its metal oxide state. Phosphorus removal by both calcined and non-calcined materials followed a pseudo-second-order kinetics. The ZnAlNO3 revealed a higher removal capacity when compared to ZnAl-LDO, which indicates that, in this case, the calcination does not improve its sorption capacity. The equilibrium study using ZnAlNO3 demonstrated that the Langmuir isotherm provided the best fit for the experimental data, with a maximum phosphorus loading capacity of 84 mg g−1. Overall, ZnAlNO3 appears to be a promising sorbent for the efficient and cost-effective removal of phosphate ions from eutrophicated water streams.

Incorporation of additives into a reinforced concrete matrix to delay or even completely avoid the initiation of corrosion during the service-life of the construction is a widely pursued topic. One of the new promising technologies achieving increased interest is to incorporate corrosion inhibitors encapsulated in layered double hydroxide (LDH). LDH structures follow a controlled release of the inhibitor while chloride is efficiently trapped at the same time. Another type of nanostructure additive offering self-protection ability in concrete is polyhedral oligomeric silsesquioxanes (POSS) developed to exhibit water-repellent functionalities protecting the reinforcement from corrosive attack. In the present laboratory work, the enhanced performance of concrete infrastructures in a marine environment was studied using a SCC design. The addition of LDH (0.5, 1 and 2 % by mass of binder (bmb)) and POSS (2 and 4 % additive level) was explored. Migration and diffusion Cl transport tests have been performed towards corrosion protection of reinforcing bars. The results showed that Cl transport decreases with the concrete maturity, and this is even more effective for concretes with LDH and POSS. This delay effect is more pronounced in the unidirectional diffusional Cl transport. LDH is significantly retarding the initiation of rebar corrosion.

In this work, the ZnAl-NO2 LDH (layered double hydroxide) is investigated as a possible additive for mitigating the chloride-induced corrosion of steel in reinforced concrete. The investigation focused on the stability and chloride binding capacity of this LDH in the pH range typical of cementitious materials. Until pH = 12.5 the material was stable and effective in capturing chloride ions from the surrounding aqueous environment. For higher pH, precisely that of hydrated cement, the LDH was partially dissolved and OH− preferentially entrapped instead of Cl−. These results suggested that ZnAl-NO2 has excellent chloride entrapping capability at neutral pH, but this is reduced with increasing pH. However, when the LDH was incorporated into mortars, the chloride ingress was delayed, signifying that the dissolution of LDH leads to a secondary mechanism responsible for chloride capture.