Roberto Herrero

Physicochemical studies of cadmium(II) biosorption by the invasive alga in Europe, Sargassum muticum.

In recent years, there has been a significant increase in the studies concerning brown seaweed as biosorbents for metal removal owing to their high binding ability and low cost. This work reports the results of a study regarding the cadmium binding equilibria of dead biomass from the seaweed Sargassum muticum; this alga is a pest fouling organism that competes with the local fucalean species and may also interfere with the “sea industry”; therefore, it would constitute an ideal material to be used as biosorbent. Seven different treatments were tested in order to obtain a stable biomass that could be suitable for industrial use under a broad range of operational conditions. The treatments employed were protonation, chemical cross‐linking with formaldehyde, KOH, Ca(OH)2 and CaCl2 or physical treatments with acetone and methanol. The equilibrium adsorption isotherms of Langmuir, Freundlich, and Langmuir–Freundlich were obtained for the quantitative description of the cadmium uptake. The effect of pH on biosorption equilibrium was studied at values ranging from 1 to 6, demonstrating the importance of this parameter for an accurate evaluation of the biosorption process. Maximum biosorption was found pH higher than 4.5. The maximum biosorption uptake for the raw biomass was 65 mg g−1, while for formaldehyde cross‐linking biomass the uptake increases to 99 mg g−1 and for protonated biomass to 95 mg g−1. Potentiometric titrations were carried out to estimate the total number of weak acid groups and to obtain their apparent pK value, 3.85, using the Katchalsky model. Kinetic studies varying cadmium concentration, algal dose, and ionic strength were carried out. Over 95% of the maximum cadmium uptake was achieved within 45 min, so the process can be considered relatively fast. A pseudo‐second‐order model, for the kinetics of cadmium biosorption, was shown to be able to reproduce experimental data points with accuracy.

Biosorption of Cadmium by Fucus spiralis

Environmental Context. Conventional processes for the removal of heavy metals from wastewaters generally involves chemical precipitation of metals (changing the pH) followed by a period to allow the metal precipitates to settle and be separated. These processes are inefficient when the metals are at a low concentration and still demand handling and disposal of toxic metal sludges. An alternative method for heavy metal removal is adsorption onto a biological material, biosorption. The biological materials, including agricultural byproducts, bacteria, fungi, yeast, and algae, all which take up heavy metals in substantial quantities, are relatively inexpensive, widely available, and from renewable sources. However, biological materials are complex and the active mechanisms often unclear. Abstract. Cadmium biosorption properties of nonliving, dried brown marine macroalga Fucus spiralis from Galician coast (northwest Spain) have been investigated. The biosorption capacity of the alga strongly depends on solution pH; the uptake is almost negligible at pH ≤ 2 and reaches a plateau at around pH 4.0. Cadmium biosorption kinetics by F. spiralis is relatively fast, with 90% of total adsorption taking place in less than one hour. A pseudo second order mechanism has been proved to be able to predict the kinetic behaviour of the biosorption process. The effect of initial cadmium ion concentration, alga dose, solution pH, and temperature on the biosorption kinetics has been studied. The Langmuir, Freundlich, Langmuir–Freundlich, and Toth isotherms were used to fit the experimental data and to find out the adsorption parameters. Acid–base properties of the alga have been studied potentiometrically in order to calculate the number of acidic groups and the apparent pK value by using Katchalsky model. The pK obtained is comparable with typical values associated to the ionization of carboxyl groups of alginates, supporting the implication of these groups in the biosorption process.

Thermodynamic and Kinetic Aspects on the Biosorption of Cadmium by Low Cost Materials: A Review

Environmental Context. The toxicity of cadmium in waters can be decreased by using a wide variety of low-cost biomaterials. A number of such investigations are reviewed here and the models used to describe the process of biosorption discussed. Fundamental investigations that probe the thermodynamics and kinetics of the biosorption process are essential for a strong understanding of all biosorption processes. Areas that still need addressing are highlighted, in particular with regard to cadmium biosorption, some models for which are ready to be tested in pilot plants. Abstract. Cadmium is internationally recognized as an important pollutant in the environment, and different methods for its removal from wastewaters (chemical precipitation being the most commonly used) have been reported in the literature. Those methods are in most cases oriented to situations with high concentrations of the pollutant. Thus, alternative removal and recovery methods are being considered for removing very low concentrations of cadmium. These methods are all based on biosorption, the passive adsorption and sequestration of metals by several natural materials of biological origin. In this review we have considered the biosorption of cadmium onto biomaterials from a physicochemical, thermodynamic, and kinetic perspective. The thermodynamic perspective is based on the characterization of the interactions of the binding sites of the biosorbents with cadmium species in aqueous solution. Traditionally, this approach has been quantified using different kinds of isotherms. In addition, the description is completed by taking into account electrostatic effects, and the influence of pH and ionic strength, which are associated with the negative charge developed, in most cases, by the biomaterial. The other point of view in this review is the kinetic one, which is necessary for a full physicochemical description of the sorbate–biosorbent system. Consequently, an updated description of the various approaches commonly employed in kinetic studies in biosorption has been carried out.

Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH2) as well as that of UQH2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH2.+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH. neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ.- resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ.- uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.

Alkanethiol monolayers and alkanethiol|phospholipid bilayers supported by mercury: an electrochemical characterization

Abstract The preparation of self-assembled alkanethiol monolayers and alkanethiolxa0∣xa0phospholipid bilayers on a home-made hanging mercury drop electrode is described. Two different experimental procedures are used to prepare alkanethiol monolayers on mercury. Once a stable thiol monolayer is obtained on the mercury electrode, formation of the bilayer is simple and highly reproducible. The bilayers consist of a thiol monolayer anchored to mercury via the sulfur atom, with a second phospholipid monolayer on top of it. Monolayers and bilayers are characterized electrochemically by measuring capacitance and resistance. Both thiol-coated and thiolxa0∣xa0lipid-coated mercury electrodes present a compact and well-ordered structure, absence of defects and high reproducibility. The permeability of these films to compounds with different degrees of lipophilicity is examined. The results point to a high impermeability of self-assembled alkanethiol monolayers and mixed alkanethiolxa0∣xa0phospholipid bilayers supported by mercury to lipophilic compounds.

Cr III binding by surface polymers in natural biomass: the role of carboxylic groups

Environmental context. Large quantities of chromium are discharged into the environment as a result of its widespread use in modern industries, and consequently, chromium could constitute a serious pollution problem. Adsorption onto natural biomass offers real potential as a way of removing chromium from the environment, because such adsorbents contain biopolymers with particular chemical stability and selectivity towards metals. In addition, natural biomass constitutes an eco-friendly and cost-effective alternative to the existing methods. Here, specific interactions between chromium and the biomass are investigated. Abstract. The chromium(III)-binding capacity of several biomaterials has been described under fixed conditions of pH (4.5) and initial metal concentration (100 mg L–1). Three of these materials (Sargassum muticum, orange peel and bracken fern) have been selected and subjected to different studies. Fourier transform infrared and scanning electron microscopy techniques were used to describe the structure of the biomaterials, supporting the hypothesis of a mechanism of metal complexation via carboxylic groups. Potentiometric titrations revealed the quantity of carboxyl groups present in S. muticum, orange peel and bracken fern: 1.78, 0.49 and 0.67 mmol g–1, respectively. Moreover, a model considering different types of binding sites was used to simulate the process and determine the apparent pK values of the main functionalities. The number of carboxylic groups was clearly correlated with the maximum amount of CrIII binding by the materials. A Langmuir competitive model was used to determine the complexation constants for chromium, log KCr, which are very close (~3), supporting the idea of the implication of essentially one acid functionality. Desorption studies were conducted for different times employing H2SO4 and sodium citrate.

A dynamic proof of mercury elimination from solution through a combined sorption-reduction process.

Physico-chemical factors affecting mercury elimination from solution using fern as sorbent have been analysed. It was demonstrated that interaction of mercury with this biomass follows two processes, adsorption by the functional groups in the biopolymers of the cell wall and reduction by easily oxidized compounds of the biomass. Batch experiments have been done to analyse the effect of pH, ionic strength of the media or competition with other metals. Ionic strength did not show a significant influence in the process, but mercury speciation with the formation of negatively charge complexes represented an important drawback in metal elimination. Continuous flow conditions were also analysed since many industrial applications will require them. These studies allowed distinguishing the two processes: mercury sorption was observed but also reduction of this metal occurs. Reduction to mercury (I) and metallic mercury has been confirmed by scanning electron microscopy analysis of the column filling after the continuous flow experiments.

Adsorptive behaviour of mercury on algal biomass: competition with divalent cations and organic compounds.

Biosorption processes constitute an effective technique for mercury elimination. Sorption properties of native and acid-treated Sargassum muticum have been studied. Effect of pH, initial mercury concentration and contact time studies provided fundamental information about the sorption process. This information was used as the reference values to analyse mercury sorption under competition conditions. Saline effect has shown little influence in sorption, when only electrostatic modifications took place upon salt addition. On the contrary, if mercury speciation dramatically changed owing to the addition of an electrolyte, such as in the case of chloride salt, very large modifications in mercury sorption were observed. Competition with other divalent cations or organic compounds has shown little or none effect on mercury, indicating that a different mechanism is taking place during the removal of these pollutants. Finally, continuous flow experiments have clearly shown that a reduction process is also taking place during mercury removal. This fact is not obvious to elucidate under batch sorption experiments. Scanning Electron Microscopy analysis of the surface of the materials show deposits of mercury(I) and metallic mercury which is indicative of the reduction process proposed.

Experimental evidences for a new model in the description of the adsorption-coupled reduction of Cr(VI) by protonated banana skin

This work reports experimental evidences, not previously considered, to evaluate the Cr(VI) removal by protonated banana skin biomass. Variations in the number of hydroxyl groups, quantified by potentiometric titrations, and CO2 evolution during experiments, were attributed mainly to the oxidation of hydroxylic entities present in the studied material. The results indicate that these groups together with the carboxylic moieties are the main functionalities involved on the adsorption-coupled reduction process. The column experiment carried out provides a new approach to obtain the maximum reduction capacity of the material (3.72 mmol g(-1)). Moreover, we hereby propose a model that reports the first evidence for the instant bound of Cr(III) species to the material used, formed after the reduction of Cr(VI) present in solution. The removal process was quantified carrying out experiments under various pHs, biomass doses and Cr(VI) concentrations, and the mechanism underlying chromium removal was identified.

Effect of ionic strength on the formal potential of the glass electrode in various saline media

We examined the variation with ionic strength (I, adjusted with KCl, KNO(3), KBr, NaCl or NaClO(4)) of the formal potential (E(const)) for glass electrodes exhibiting a Nernstian response (i.e. E(cell)=E(const)-slog[H(+)]). For this purpose, we investigated the different factors included in the formal potential, so we obtained reported values for the liquid junction potential as a function of ionic strength and determined the logarithm of the activity coefficient for the proton in various saline media, using Pitzer equations.