Heavy Metal Removal from Wastewaters by Biosorption: Mechanisms and Modeling

Potential use of Ulva intestinalis-derived biochar adsorbing phosphate ions in the cultivation of winter wheat Tristicum aestivum

In this work, the properties of biochar produced from green macroalga Ulva intestinalis by pyrolysis were studied at temperatures of 300, 500, and 700 °C. This biochar was characterized in terms of multielemental composition, BET surface area, total pore volume, and biosorption properties toward phosphate ions. Biochar produced at 700 °C-25 m2/g had the highest surface area. The kinetics and isotherms of sorption processes of phosphate ions as sorbate by these sorbents were investigated. Modified biochar was able to remove 84.3% of phosphate ions from wastewater, whereas non-modified biochar-only 40.6%. Hence, biochar enriched with phosphate ions can serve as a valuable soil amendment. Pot experiments performed on winter wheat (Triticum aestivum) with a 3% addition of dry Ulva intestinalis, pristine biochar, and Mg-modified biochar enriched with phosphate ions showed that these amendments stimulated plant growth (length and fresh weight of plants) as well as enlarging the chlorophyll content in leaves. Our results indicate that the production of biochar (pristine and Mg-impregnated) is a sustainable option to valorize the biomass of seaweeds, and to recycle phosphorus from wastewater.

A Mathematical Study of Metal Biosorption on Algal-Bacterial Granular Biofilms

A multiscale mathematical model describing the metals biosorption on algal-bacterial photogranules within a sequencing batch reactor (SBR) is presented. The model is based on systems of partial differential equations (PDEs) derived from mass conservation principles on a spherical free boundary domain with radial symmetry. Hyperbolic PDEs account for the dynamics of sessile species and their free sorption sites, where metals are adsorbed. Parabolic PDEs govern the diffusion, conversion and adsorption of nutrients and metals. The dual effect of metals on photogranule ecology is also modelled: metal stimulates the production of EPS by sessile species and negatively affects the metabolic activities of microbial species. Accordingly, a stimulation term for EPS production and an inhibition term for metal are included in all microbial kinetics. The formation and evolution of the granule domain are governed by an ordinary differential equation with a vanishing initial value, accounting for microbial growth, attachment and detachment phenomena. The model is completed with systems of impulsive differential equations describing the evolution of dissolved substrates, metals, and planktonic and detached biomasses within the granular-based SBR. The model is integrated numerically to examine the role of the microbial species and EPS in the adsorption process, and the effect of metal concentration and adsorption properties of biofilm components on the metal removal. Numerical results show an accurate description of the photogranules evolution and ecology and confirm the applicability of algal-bacterial photogranule technology for metal-rich wastewater treatment.

Heavy metal tolerance in microalgae: Detoxification mechanisms and applications

The proficiency of microalgae to resist heavy metals has potential to be beneficial in resolving various environmental challenges. Global situations such as the need for cost-effective and ecological ways of remediation of contaminated water and for the development of bioenergy sources could employ microalgae. In a medium with the presence of heavy metals, microalgae utilize different mechanisms to uptake the metal and further detoxify it. Biosorption and the next process of bioaccumulation are two such major steps and they also include the assistance of different transporters at different stages of heavy metal tolerance. This capability has also proved to be efficient in eradicating many heavy metals like Chromium, Copper, Lead, Arsenic, Mercury, Nickel and Cadmium from the environment they are present in. This indicates the possibility of the application of microalgae as a biological way of remediating contaminated water. Heavy metal resistance quality also allows various microalgal species to contribute in the generation of biofuels like biodiesel and biohydrogen. Many research works have also explored the capacity of microalgae in nanotechnology for the formation of nanoparticles due to its relevant characteristics. Various studies have also revealed that biochar deduced from microalgae or a combination of biochar and microalgae can have wide applications specially in deprivation of heavy metals from an environment. This review focuses on the strategies adopted by microalgae, various transporters involved in the process of tolerating heavy metals and the applications where microalgae can participate owing to its ability to resist metals.

High Adsorption of Hazardous Cr(VI) from Water Using a Biofilter Composed of Native Pseudomonas koreensis on Alginate Beads

Most conventional methods to remove heavy metals from water are efficient for high concentrations, but they are expensive, produce secondary pollution, and cannot remove low concentrations. This paper proposes a biological system to remove Cr(VI) from aqueous solutions; the biofilter is composed of a native Pseudomonas koreensis immobilized in calcium alginate beads. Lab experiments were conducted in batch reactors, considering different operating conditions: Cr(VI) concentration, temperature, pH, and time. At 30 °C and a pH of 6.6, the immobilized bacteria achieved their optimal adsorption capacity. In the chromium adsorption system, saturation was reached at 30 h with a qmax = 625 mg g-1. By adjusting the experimental data to the Langmuir and Freundlich models, it is suggested that P. koreensis forms a biofilm with a homogeneous surface where Cr(VI) is adsorbed and that the bacteria also incorporates the metal in its metabolism, leading to a multilayer adsorption. On the other hand, using Fourier transform infrared spectroscopy, it was inferred that the functional groups involved in the adsorption process were O-H and C=O, which are a part of the P. koreensis cell wall.

Submerged zone and vegetation drive distribution of heavy metal fractions and microbial community structure: Insights into stormwater biofiltration system

Biofiltration system is a widely used stormwater treatment option that is effective in removing heavy metals. The concentration and distribution of heavy metal fractions in biofiltration filter media, as well as the microbiota composition affected by the design parameters, are relatively novel concepts that require further research. A laboratory-scale column study was conducted to investigate the microbial community and the fractionation of heavy metals (Pb, Cu, Cr, and Cd) extracted from filter media samples, subjected to the presence of vegetation, submerged zone (SZ), and major environmental parameters (pH, water content). Sequential extractions revealed that, compared to the three other fractions (exchangeable fraction, reducible fraction, and oxidizable fraction), the residual fraction was the most represented for each metal (41 - 82 %). As a result, vegetation was found to reduce pH value, and significantly decrease the concentration of the exchangeable fraction of Pb in the middle layer, and the oxidizable fraction of Pb, Cu, Cd, and Cr in the middle and bottom layers (p < 0.05). The formation of an anoxic environment by submerged zone settlements resulted in a significant decrease in the concentration of reducible fractions and a significant increase in the concentration of oxidizable fractions for four heavy metals (p < 0.05). In addition, the analysis of the microbiota showed that the diversity and richness of microorganisms increased in the presence of SZ and plants. The dominant phylum in biofiltration was Proteobacteria, followed by Firmicutes, Bacteroidetes, Acidobacteria, and Actinobacteria as major phyla. Heavy metal fractions could regulate the structure of microbial communities in biofiltration. The findings of this study would enrich our understanding of the improvement of multi-metal-contaminated runoff treatment and highlight the impact of design parameters and heavy metal fractionation on microbial community structure in the biofiltration system.

New energy approaches to the use of waste biosorbents of microalgae Chlorella kessleri (Chlorellaceae, Chlorellales)

The use of microalgae Chlorella kessleri VKPM A1-11 ARM (RF, NPO Algobiotechnology) for environmental and energy purposes is considered. The results of our study of the use of C. kessleri microalgae biomass as a biosorbent to purify model wastewater from Cu2+ ions under static conditions are presented. Biosorption is a promising technology for the treatment of industrial effluents containing various heavy metal compounds, but the issues of economic benefits of using biosorbents, their environmental safety and the cost of disposal of used sorbents are subject to much discussion. The paper proposes to dispose the used biosorbent formed after wastewater treatment from copper as an additional fuel. The copper concentration in the filtrate was determined by colorimetric analysis with sodium diethyldithiocarbamate. The cleaning efficiency and sorption capacity of the dry mass of C. kessleri were obtained by calculation. The maximum sorption capacity for Cu2+ ions was 4.2 mg/g. The purification efficiency reached 87% at the initial concentration of Cu2+ ions being 97 mg/l. Tests to estimate the specific heat of combustion of C. kessleri biomass and used biosorbents based thereon were carried out by the calorimetric method using a bomb calorimeter. The specific heats of combustion were 22,125 kJ/kg and 21,674 kJ/kg, respectively. A comparison of these values with traditional energy carriers is given. A technological scheme has been developed for a waste-free cycle of using C. kessleri to treat wastewater from industrial enterprises with the production of several valuable resources as end products, such as purified water, energy resources, fertilizers, and recycled metals. The obtained results of our study can be applied in technologies for post-treatment of wastewater from various industrial enterprises using biological non-waste resources.

Synergy of production of value-added bioplastic, astaxanthin and phycobilin co-products and Direct Green 6 textile dye remediation in Spirulina platensis

Phyco-remediation of dyestuffs in textile wastewaters is of economic, industrial, and environmental importance. We evaluated the remediation of the textile dye, Direct Green 6 (DG6), by Spirulina platensis, and investigated the novel possibility that DG6 treatment enhances production of the biopolymer, polyhydroxybutyrate (PHB). We showed that both live and dead cells of Spirulina were capable of DG6 remediation, but live cells could be re-used with no loss of remediation efficiency. Furthermore, DG6 remediation by live cells resulted in increased algal biomass and trichome lengths, and stimulated production of valuable metabolites, including PHB, antioxidants, carbohydrates and pigments (phycobilins and astaxanthin). We determined the optimal conditions for DG6 remediation and an artificial neural network (ANN) accurately modeled the experimental data and predicted the concentration of dye as the most and algal turbidity as the least important parameters for DG6 removal efficiency. A DG6 concentration of 60 mg L^-1 resulted in the highest simultaneous co-production of PHB (12.7 ± 1.7% DW) and increase of astaxanthin (194%), carotenoids (50%), phenol (51%), carbohydrates (27%) total phycobilin (43%), together with the enhancement of biomass and trichome lengths (95%). Oxidative stress indices and enzyme activities such as peroxidases and laccase (involved in dye removal/antioxidant functions) were also increased by dye dosage. On the basis of our results, we propose that S. platensis may use DG6 dye as a nitrogen/carbon source for co-accumulation of valuable bioplastic and metabolites.

Insights into nanomycoremediation: Secretomics and mycogenic biopolymer nanocomposites for heavy metal detoxification

Our environment thrives on the subtle balance achieved by the forever cyclical nature of building and rebuilding life through natural processes. Fungi, being the evident armor of bioremediation, is the indispensable element of the soil food web, contribute to be the nature's most dynamic arsenal with non-specific enzymes like peroxidase (POX), glutathione peroxidase (GPx), catalase (CAT), superoxide dismutase (SOD), non-enzymatic compounds like thiol (-SH) groups and non-protein compounds such as glutathione (GSH) and metallothionein (MT). Recently, the area of nanomycoremediation has been gaining momentum as a powerful tool for environmental clean-up strategies with its ability to detoxify heavy metals with its unique characteristics to adapt mechanisms such as biosorption, bioconversion, and biodegradation to harmless end products. The insight into the elaborate secretomic processes provides us with huge opportunities for creating a magnificent living bioremediation apparatus. This review discusses the scope and recent advances in the lesser understood area, nanomycoremediation, the state-of-the-art, innovative, cost-effective and promising tool for detoxification of heavy metal pollutants and focuses on the metabolic capabilities and secretomics with nanobiotechnological interventions.