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This paper reviews some practical aspects of the application of algal biomass for the biosorption of heavy metals from wastewater. The ability of different algal species to remove metals varies with algal group and morphology, with the speciation of specific metals and their competition with others in wastewater, and with environmental or process factors. The scattered literature on the uptake of heavy metals by both living and dead algal biomass -both macroalgae and immobilized microalgae -has been reviewed, and the uptake capacity and efficiency of different species, as well as what is known about the mechanisms of biosorption, are presented. Data on metal uptake have commonly been fitted to equilibrium models, such as the Langmuir and Freundlich isotherm models, and the parameters of these models permit the uptake capacity of different algal species under different process conditions to be compared. Higher uptake capacities have been found for brown algae than for red and green algae. Kelps and fucoids are the most important groups of algae used for biosorption of heavy metals, probably because of their abundant cell wall polysacchrides and extracellular polymers. Another important practical aspect is the possibility of re-using algal biomass in several adsorption/desorption cycles (up to 10 have been used with Sargassum spp), and the influence of morphology and environmental conditions on the re-usability of algal tissue is also considered.

The transformation of ferrihydrite (5Fe 2 O 3 3 9H 2 O) to hematite (R-Fe 2 O 3 ) under alkaline condition in the presence and absence of lead was for the first time investigated using in situ, time-resolved synchrotron-based energy dispersive X-ray diffraction combined with off-line chemical characterization and imaging. The results showed that the crystallization of hematite occurred via a two-stage process with goethite (R-FeOOH) as an intermediate phase. The presence of lead enhanced the formation of hematite and reduced the induction times (∼20-30%) but had little effect on the mechanism of the transformation reactions. The reaction rates for the two systems (with and without lead) ranged from 12 to 259 Â 10 -4 s -1 and 19 to 461 Â 10 -5 s -1 for the first and second stage, respectively. The activation energies of nucleation of the two systems were 16((3) and 9((2) kJ/mol, while the activation energies for crystallization ranged from 41((7) to 77(( 14) kJ/mol. During the hematite crystallization, the majority of the lead in the system was rapidly and irreversibly incorporated into the final hematite, while only minor amounts of lead were released back into solution.

24In this study we used in-situ synchrotron-based energy dispersive X-ray diffraction 25 (EDXRD) to follow the transformation of ferrihydrite to hematite at pH ~ 8 and ionic strength 26 between 0.1 and 0.7. In addition, the effects of co-precipitated molybdenum (Mo) and 27 vanadium (V) on the transformation were evaluated both through EDXRD and X-ray 28 absorption spectroscopy (XAS). The transformation end-product in all experiments was 29 hematite with small amounts of goethite as an intermediate phase. XAS results revealed that 30Mo and V were initially adsorbed and co-precipitated onto/with ferrihydrite as molybdate and 31 vanadate ions, respectively. After the ferrihydrite transformed to hematite these metals were 32 sequestered into the hematite structure. The kinetic results showed that the presence of Mo 33 and V in the ferrihydrite structure had little to no effect on the kinetics of the ferrihydrite 1 transformation. The transformation however occurred ~ 30% faster at higher ionic strength. 2 3

This study aims to highlight discrepancies between experimental and simulation linked to the mechanisms of Mo and V adsorption onto ferrihydrite (FHY) nanoparticles. We have measured adsorption capacities and uptake efficiencies and then fitted and compared these with outputs from various geochemical and adsorption models that were run as a function of pH, surface area (SA) and ferrihydrite particles size distributions. Our results revealed that the experimental data for the Mo system could be fitted very well, but this was not the case for the V system, when a model default value for the SA of FHY of 600 m2 g−1 was used. The discrepancy in the results for the V system can be explained by the lack of specific V species and/or associated constants in databases and variation in software versions, which change the outputted chemical species. Our comparative results also confirm that any experimental variables used as modelling inputs need to be checked carefully prior to any modelling exercises.

In this paper, the kinetics of Mo and V (100 mM) uptake on ferrihydrite (FHY) were evaluated in batch, mono-sorbate systems at pH between 4 and 9, and in bi-sorbate systems in the presence of P (100 mM) at pH 7. In the Mo and V single-sorbate experiments, 100% adsorption was observed at pH values below 6 and 8, respectively. Above the point of zero charge (PZC = 7.97) of FHY, the adsorption efficiency for Mo dropped dramatically (20% at pH 8) while V showed high uptake efficiencies even at pH 9 (60% efficiency). The results from the bi-sorbate experiments (Mo-P and V-P) showed that at pH 7, P out-competed (97%) Mo for binding sites on FHY, while in the V-P binary system only~44% of the binding sites are occupied by P with the remaining sites being occupied by V.

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