Manuel E. Sastre de Vicente

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.

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.

Surface charge and permeable gel descriptions of the ionic strength influence on proton binding to seaweed biomass

Abstract The development of physicochemical models for the description of cation binding to algal biomass is an essential issue for the practical application of seaweeds as biosorbents in waste water treatment. These models should be able to account for the polyelectrolytic properties of these materials, and to reasonably describe and predict the ionic strength effect. Data regarding the proton binding equilibria of Sargassum muticum, Cystoseira baccata and Saccorhiza polyschides in NaCl and KNO3 were reported on a preceding paper. These data are re-analysed in this work, in order to compare two different electrostatic models (surface charge and Donnan models), representing two opposite idealised interface structures (planar surface and permeable gel). Simple analytical equations relating the apparent pK with both the dissociation degree and the ionic strength are derived for each case. The experimental data are reproduced with similar accuracy by the surface charge and the simplest Donnan model (rigid volume), although the latter seems to yield slightly better results. The comparison of the geometric parameters involved in each model (Donnan volume and specific area) with independent experimental measurements is not straightforward. Therefore, both models seem to be almost equivalent, within their respective assumptions. Nevertheless, Donnan has the advantage that provides a simple way to account for activity coefficient effects and non-specific binding.

Aluminium removal from wastewater by refused beach cast seaweed. Equilibrium and dynamic studies.

Aluminium removal has been investigated in synthetic and real wastewaters provided by an aluminium surface treatment plant. Marine algae, obtained as beach cast seaweed (a refuse substance) were used as adsorption material. The influence of pH, metal concentration and time for aluminium elimination was studied by use of synthetic solutions. The optimum pH value was 4.0, which provided a maximum adsorption capacity of 22.5 mg g(-1). The adsorption percentage surpassed 80% in less than 30 min of contact time. Real solutions from the industrial unit were fully characterized and tested in two different fixed-bed columns. One column was filled with 27.5 g of dried beach cast seaweed. Three cycles of adsorption and two of desorption were carried out. The first cycle (12 mg g(-1) maximum sorption capacity) was enough to reach the maximum adsorption capacity at 15 mL min(-1) flow rate. The second column was packed with 1100 g of seaweed and its behaviour was compared to another column filled with activated charcoal, following both the same experimental procedure. Maximum sorption capacity was 14 mg g(-1) for seaweed, whereas the activated charcoal only reached 1.6 mg g(-1) (flow rate of 250 mL min(-1)).

Physicochemical characterisation of the ubiquitous bracken fern as useful biomaterial for preconcentration of heavy metals.

Batch experiments with dry bracken fern have been done to determine cadmium and lead sequestering capacity of this biomaterial. Biomass characterisation was done by infrared spectroscopy and potentiometric analysis. The effect of pH of the metal containing solution, contact time and initial metal concentration has been studied, together with the acid-base properties of the biomaterial. Results obtained have been analysed using mathematical and modelling techniques. Effect of pH on metal sequestration has been correlated with observed acid-base properties of the natural substrate. Kinetic data analysis provided relevant information about metal sequestration rate, showing important differences between lead and cadmium. Maximum uptake was found to be the same for both metals 0.410 mmol/g. This value was also clearly correlated to the number of acidic groups determined for this material which was found to be 0.432 mmol of acidic groups per gram of fern. Results obtained indicate that acidic groups are the functional groups responsible of the sequestration of metal ions and that bracken fern is a promising material for metal preconcentration.