Reduced Chitosan as a Strategy for Removing Copper Ions from Water.

Toxic heavy metals are priority pollutants in wastewater, commonly present in dangerous concentrations in many places across the globe. Although in trace quantities copper is a heavy metal essential to human life, in excess it causes various diseases, whereby its removal from wastewater is a necessity. Among several reported materials, chitosan is a highly abundant, non-toxic, low-cost, biodegradable polymer, comprising free hydroxyl and amino groups, that has been directly applied as an adsorbent or chemically modified to increase its performance. Taking this into account, reduced chitosan derivatives (RCDs 1-4) were synthesised by chitosan modification with salicylaldehyde, followed by imine reduction, characterised by RMN, FTIR-ATR, TGA and SEM, and used to adsorb Cu(II) from water. A reduced chitosan (RCD3), with a moderate modification percentage (43%) and a high imine reduction percentage (98%), proved to be more efficient than the remainder RCDs and even chitosan, especially at low concentrations under the best adsorption conditions (pH 4, RS/L = 2.5 mg mL-1). RCD3 adsorption data were better described by the Langmuir-Freundlich isotherm and the pseudo-second-order kinetic models. The interaction mechanism was assessed by molecular dynamics simulations, showing that RCDs favour Cu(II) capture from water compared to chitosan, due to a greater Cu(II) interaction with the oxygen of the glucosamine ring and the neighbouring hydroxyl groups.

Silica Aerogels/Xerogels Modified with Nitrogen-Containing Groups for Heavy Metal Adsorption.

Heavy metals are common inorganic pollutants found in the environment that have to be removed from wastewaters and drinking waters. In this work, silica-derived aerogels and xerogels were modified via a co-precursor method to obtain functional adsorbents for metal cations. A total of six formulations based upon four different functional precursors were prepared. The materials' structural characterization revealed a decreased porosity and surface area on modified samples, more prominent in xerogel counterparts. Preliminary tests were conducted, and the prepared samples were also compared to activated carbon. Three samples were selected for in-depth studies. Isotherm studies revealed that the pre-selected samples remove well copper, lead, cadmium and nickel, and with similar types of interactions, following a Langmuir trend. The adsorption kinetics starts very fast and either equilibrium is reached quickly or slowly, in a two-stage process attributed to the existence of different types of active sites. Based on the previous tests, the best sample, prepared by mixing different functional co-precursors, was selected and its behavior was studied under different temperatures. For this material, the adsorption performance at 20 °C is dependent on the cation, ranging from 56 mg·g-1 for copper to 172 mg·g-1 for lead.

Assessment of heavy metal pollution from anthropogenic activities and remediation strategies: A review.

Heavy metal pollution is a nefarious issue with implications for life. Heavy metals are natural occurring elements, having both natural and anthropogenic sources. The latter are however the most significant, releasing greater amounts of these pollutants in more toxic and mobile forms. Their chemistry and dynamics in the ecosystems are presented, and the relation to the pollution problematic thereof is discussed. The concentration of heavy metals in several sites, assessed in water, soil and sediment samples, affected by different pollution sources are reviewed. These evidence how human activities impact natural media and how the pollution spreads. The pollution in each media is assessed by the concentration relative to drinking and irrigation water guidelines, and by the geoaccumulation index of soils and sediments. It is found that ore extraction and processing and metallurgical industries stand atop the most polluting sources. Given the dynamics of heavy metal cations and that, most of these are released in liquid effluents, wastewater treatment techniques for the removal of heavy metals are also surveyed and critically discussed. Economic viability at a large municipal scale and the ability to comply with strict regulations are the determining factors in the selection of these techniques. A critical discussion on the viability of such techniques is made, reviewing some literature studies and commenting on their applicability on the previously found polluted media.

Amine Modification of Silica Aerogels/Xerogels for Removal of Relevant Environmental Pollutants.

Serious environmental and health problems arise from the everyday release of industrial wastewater effluents. A wide range of pollutants, such as volatile organic compounds, heavy metals or textile dyes, may be efficiently removed by silica materials advanced solutions such as aerogels. This option is related to their exceptional characteristics that favors the adsorption of different contaminants. The aerogels performance can be selectively tuned by an appropriate chemical or physical modification of the aerogel's surface. Therefore, the introduction of amine groups enhances the affinity between different organic and inorganic contaminants and the silica aerogels. In this work, different case studies are reported to investigate and better understand the role of these functional groups in the adsorption process, since the properties of the synthesized aerogels were significantly affected, regarding their microstructure and surface area. In general, an improvement of the removal efficiency after functionalization of aerogels with amine groups was found, with removal efficiencies higher than 90% for lead and Rubi Levafix CA. To explain the adsorption mechanism, both Langmuir and Freundlich models were applied; chemisorption is most likely the sorption type taking place in the studied cases.

Chitosan-Silica Composite Aerogel for the Adsorption of Cupric Ions.

A chitosan-silica hybrid aerogel was synthesized and presented as a potential adsorbent for the purification of cupric ion-contaminated media. The combination of the organic polymer (chitosan), which can be obtained from fishery wastes, with silica produced a mostly macroporous material with an average pore diameter of 33 µm. The obtained aerogel was extremely light (56 kg m-3), porous (96% porosity, 17 cm3 g-1 pore volume), and presented a Brunauer-Emmett-Teller surface area (SBET) of 2.05 m2 g-1. The effects of solution pH, aerogel and Cu(II) concentration, contact time, and counterion on cupric removal with the aerogel were studied. Results showed that the initial pH of the cation-containing aqueous solution had very little influence on the removal performance of this aerogel. According to Langmuir isotherm, this material can remove a maximum amount of ca. 40 mg of cupric ions per gram and the kinetic data showed that the surface reaction was the rate-limiting step and equilibrium was quickly reached (in less than one hour). Thus, the approach developed in this study enabled the recovery of waste for the preparation of a novel material, which can be efficiently reused in a new application, namely water remediation.

Ligands as copper and nickel ionophores: Applications and implications on wastewater treatment.

Modern society depends on many finite natural resources, from which metals are of great importance. Copper and nickel's relevance is due to their vast applications, resulting in high market value and demand. As such, their polluting emissions are also significant and their removal from wastewaters is imperative. Moreover, effluent treatment techniques can be used to recover the metallic cations, via selective processes. In this review, copper and nickel selective ligands in the literature are surveyed. These are most commonly Schiff bases, along with crown ethers and porphyrins. They are usually employed in ion sensing (colorimetric chemosensors or electrodes) with great success - the disruption in response of colorimetric sensors is up to 7% and binding constants are usually at least one order of magnitude greater with the desired cation than with interferents. However, modified adsorbents are also reported. The possibilities of using ionophores in wastewater cleaning, allowing the treatment of effluents and the selective recovery of valuable materials, and their implications on new green policies is discussed.

Efficient adsorption of multiple heavy metals with tailored silica aerogel-like materials.

Recently developed tailored adsorbents for heavy metal uptake are studied in batch tests with Cu, Pb, Cd, Ni, Cr and Zn, in order to decontaminate polluted environments where these heavy metals are found in solution - water courses and groundwater. The adsorbents feature mercapto or amine-mercapto groups that are capable of complexating the cations. Through the use of equilibrium tests it is found that a remarkably high heavy metal uptake is obtained for all metals (ranging from 84 to 140 mg/g). These uptake values are quite impressive when compared to other adsorbents reported in the literature, which is also due to the double functionalization present in one of the adsorbents. For the best adsorbent, adsorption capacities followed the order Cu(II) > Pb(II) > Zn(II) > Cr(III) > Cd(II) > Ni(II). With these adsorbents, the removal process was fast with most of the metals being removed in less than 1 h. Competitive sorption tests were performed in tertiary mixtures that were based on real world polluted sites. It was found that although competitive sorption occurs, affecting the individual removal of each metal, all the cations in solution still interact with the adsorbent, achieving removal values that make this type of material very interesting for its proposed application.

Effect of different types of surfactants on the microstructure of methyltrimethoxysilane-derived silica aerogels: A combined experimental and computational approach.

HYPOTHESIS: Surfactants interfere with sol-gel particle/pore growth, influencing the structure and properties of silica aerogels. Their ability to induce microscopic changes in the aerogel's structure may be useful to improve/control the thermal insulation performance of aerogels. EXPERIMENTS: The influence of different types of surfactants (anionic, cationic and non-ionic) on the microstructural arrangement and macroscopic properties of methyltrimethoxysilane (MTMS)-based aerogels was evaluated for the first time, using an experimental and computational comparative approach. Molecular dynamics simulations were performed based on two representative silica molecular structures derived from MTMS, while the experimentally-obtained silica aerogels were characterized in terms of chemical/structural/mechanical/thermal insulation properties. FINDINGS: The use of both hexadecyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS) led to a decrease in bulk density, thermal conductivity and average pore size of the aerogels, with notorious increase of their flexibility. The observed changes were due to microstructural arrangements, as evidenced by scanning electron microscopy (SEM). However, the non-ionic surfactant, Pluronic F-127, did not have a positive impact on the desired properties. Globally, the simulation results support the experimental findings, suggesting differentiated microstructural changes induced by the use of cationic or anionic surfactants. The addition of CTAB and SDS generally resulted in smaller or larger silica aggregates, respectively.

Heavy metals in Iberian soils: Removal by current adsorbents/amendments and prospective for aerogels.

Heavy metals are dangerous pollutants that in spite of occurring naturally are released in major amounts to the environment due to anthropogenic activities. After being released in the environment, the heavy metals end up in the soils where they accumulate as they do not degrade, adversely affecting the biota. Because of the dynamic equilibria between soil constituents, the heavy metals may be present in different phases such as the solid phase (immobilized contaminants) or dissolved in soil solution. The latter form is the most dangerous because the ions are mobile, can leach and be absorbed by living organisms. Different methods for the decontamination of polluted soils have been proposed and they make use of two different approaches: mobilizing the heavy metals, which allows their removal from soil, or immobilization that maintains the metal concentrations in soils but keeps them in an inert form due to mechanisms like precipitation, complexation or adsorption. Mobilization of the heavy metals is known to cause leaching and increase plant uptake, so this treatment can cause greater problems. Aerogels are incredible nanostructured, lightweight materials with high surface area and tailorable surface chemistry. Their application in environmental cleaning has been increasing in recent years and very promising results have been obtained. The functionalization of the aerogels can give them the ability to interact with heavy metals, retaining the latter via strong adsorptive interactions. Thus, this review surveys the existing literature for remediation of soils using an immobilization approach, i.e. with soil amendments that increase the soil sorption/retention capacity for heavy metals. The considered framework was a set of heavy metals with relevance in polluted Iberian soils, namely Cd, Cr, Cu, Ni, Pb and Zn. Moreover, other adsorbents, especially aerogels, have been used for the removal of these contaminants from aqueous media; because groundwater and soil solution have dynamic equilibria with the soil solid phase, these works allowed to draw conclusions and perspectives for the use of aerogels not only as adsorbents in aqueous media but also as amendments for the remediation of heavy metal polluted soils.