Special Issue on Biofilm Engineering for Heavy-Metal Removal and Recovery

Adsorption is a highly effective physicochemical process for removing heavy metals from wastewater, especially at low initial metal concentrations. Agricultural by-products such as rice husk, coconut husk, grounded coconut shells, saw dust, neem leaves and barks, sugarcane baggase, as well as waste materials such as sludge, fly ash, and bottom ash have been reported in this special issue for the removal of metals from wastewater.

Manikandan et al. investigated Cr(VI) removal in batch and continuous mode using dry Litchi chinensis as the adsorbent. In batch mode, the rate of Cr(VI) sorption by Litchi peels was very high with most of the metal sorption (94.5%) occurring within the first 1 h of experimentation. Results from the continuous column study showed a maximum mass Cr(VI) loading of 29,160 and 10,437 mg at a flow rate of 2.5 and $5\mathrm{ml}/\min$, respectively. Furthermore, the sorbent regeneration followed by Cr(VI) biosorption demonstrated that litchi peels could be reused for at least three consecutive cycles without any significant loss in its Cr(VI) biosorption capacity. Rajamohan and Rajasimman reported the influence of process variables, such as initial solution pH, adsorbent dose, initial mercury concentration, agitation speed, and temperature on mercury removal from aqueous solution using raw activated sludge as the adsorbent. These authors fitted the experimental data to well-known isotherm models reported in the literature, and they observed that the Langmuir isotherm accurately fitted the equilibrium data, confirming that the process followed the monolayer sorption theory. The maximum uptake capacity obtained was $57.8\mathrm{mg}/\mathrm{g}$ at an initial mercury concentration of $100\mathrm{mg}/\mathrm{L}$ at 35°C and 180 min of contact time.

Uwamariya et al. used iron oxide-coated sand (IOCS) and granular ferric hydroxide (GFH) to study the effect of Ca and pH on the removal of Cu and Cd from groundwater by adsorption. From their results, it is clear that IOCS showed a higher removal efficiency for both Cu and Cd than GFH; however, the groundwater pH determined whether adsorption and/or precipitation played a major role in the removal of these two metal ions. Anew, the authors also showed that Ca competes with Cu and Cd for the same adsorption sites on IOCS and GFH, thereby reducing their adsorption capacity. Krupa et al. showed that a river sand-based biobarrier is very effective for the removal of organic pollutants and heavy metals from landfill leachate. Bacillus insolitus, a lead-tolerant bacterium, was isolated from landfill leachate and used as the inoculum in the sand barrier. Based on the continuous study carried out for 36 days, the authors reported that the lead-enriched cultures showed higher chemical oxygen demand (COD) and lead removal efficiencies (62 and 28%, respectively) compared to that of nonenriched cultures (57 and 13%, respectively).

Dutta et al. used coffee grounds and wheat straw to study the adsorption kinetics of Cd from aqueous solutions. The authors performed several batch tests to elucidate the effect of pH, adsorbent concentration, contact time, and initial metal concentration on the removal of Cd. Their results showed that equilibrium conditions were achieved in less than 30 min for the tested adsorbents and that both physisorption and chemisorption mechanisms controlled the adsorption performance. Cheng et al. tested dried biomass waste from biotrickling filters as an adsorbent for the removal of Pb(II) ions from aqueous solutions. The results from their study showed that the Pb(II) sorption capacity increased from 19 to $86\mathrm{mg}/\mathrm{g}$ with an increase in initial pH from 2.0 to 5.0. Besides, the Pb(II) sorption capacity increased with an increase in initial Pb(II) concentration. By fitting the experimental data to the Langmuir’s isotherm model, the monolayer adsorption capacity of the dried biomass was found to be $160\mathrm{mg}/\mathrm{g}$ for Pb(II) ions. Besides, the authors evaluated the thermodynamic parameters such as Gibb’s free energy change ($\mathrm{\Delta }\mathrm{G}°$), enthalpy change ($\mathrm{\Delta }\mathrm{H}°$), and entropy change ($\mathrm{\Delta }\mathrm{S}°$) as a function of different operational temperatures to evaluate the feasibility of applying this adsorption process under different environmental conditions.

Some of the currently used techniques for the treatment of metal-contaminated water have limitations for practical use due to high operational costs, inconsistency in maintaining high performances, requirement of high land areas, and other constraints from environmental regulations. With recent scientific advancements, researchers have been looking at the possibility of integrating different technologies, for instance a combination of chemical and biological techniques, to achieve high metal-removal efficiencies.

Sikirman and Krishnan investigated the surface modification of titanium dioxide (${\mathrm{TiO}}_2$) by codoping with nitrogen and iron to synthesize a novel photocatalyst that was found to be active under visible light. The authors studied the application of this photocatalyst for the degradation of methylene blue and the results showed that a 1.0% N and 1.0% Fe-codoped ${\mathrm{TiO}}_2$ photocatalyst yielded a maximum of 80% methylene blue removal within 5 h of irradiation time. Basumatary et al. prepared MCM-48, a ceramic composite membrane on a low-cost ceramic support by a hydrothermal treatment method for the separation of Cr(VI) from an aqueous solution. The synthesized MCM-48 powder and composite membrane were characterized by X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), ${\mathrm{N}}_2$ adsorption/desorption isotherms, and field emission scanning electron microscope (FESEM). According to the results, the highest rejection of 81% was observed at a feed concentration of 1000 parts per million (ppm) and an applied pressure of 207 kPa.

D’Acunto et al. developed a mathematical model to describe the adsorption process of heavy metals onto biofilms. The authors used nonlinear hyperbolic equations for biomass and extracellular polymeric substances (EPS) growth and parabolic partial differential equations for substrate evolution. Accordingly, the model was able to describe biofilm growth dynamics including spatial distribution of microbial species, substrate concentrations, EPS formation, and, in particular, it was able to simulate heavy-metal diffusion and adsorption into biofilm.

Kim et al. investigated the effect of hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) flow rate and sodium hydroxide (NaOH) concentration on the gas-liquid phase mass transfer coefficient during absorption of ${\mathrm{H}}_2\mathrm{S}$ onto NaOH solution and subsequent synthesis of sodium hydrosulfide (NaSH). The results of industrial scale experiments carried out by the authors revealed that this process produces NaSH at the rate of $30\mathrm{t}/\mathrm{day}$, which is presently being used in zinc, lead, and copper smelter wastewater treatment plants in the Ulsan industrial complex, South Korea. Owing to this large-scale production, an industrial symbiosis network has evolved spontaneously within the Ulsan EcoIndustrial Park (EIP) region. In another paper, biomineralization of nickel, copper, lead, cobalt, zinc, cadmium, and calcium by T. tumescens and mineral precipitations in both liquid and porous media were investigated by Li et al. T. tumescens catalyzed hydrolysis of urea was used to produce calcium carbonate crystals, which can bind sand particles to give chemical and physical properties similar to those of sandstone. The results showed that T. tumescens when added into different heavy-metal containing solutions, it could induce the crystal formation and remove heavy-metal contaminants in wastewater and soil.

Janyasuthiwong et al. studied the effect of pH on the performance of sulfate and thiosulfate-fed sulfate-reducing inverse fluidized bed (IFB) reactors and modeled the performance of the IFB reactor using artificial neural networks (ANN). By using sulfate as the electron acceptor, the average COD removal efficiency was 75 and 58% at pH 7.0 and 5.0, respectively, whereas the sulfate removal efficiency was 74.4 and 50.4%, respectively. The results obtained from an ANN model showed that a network topology of 3-7-3 gave good predictions of the reactor’s performance. Based on a sensitivity analysis, the authors ascertained that pH was the most dominant factor that affected the reactor’s long-term performance. Yogeswari et al. conducted batch experiments to investigate the role of the iron concentration on hydrogen production from confectionery wastewater. A maximum cumulative hydrogen production of $2475\mathrm{mL}$ was obtained under the following optimized conditions: $700\mathrm{mg}\mathrm{COD}/\mathrm{L}$, pH 5.5, and $0.15\mathrm{g}/\mathrm{L}$ iron concentration. Furthermore, using 16S rRNA sequencing and nucleotide basic local alignment search tool (BLASTN) identification, the authors confirmed the presence of Corynebacterium kutscheri YMAU-13 (KF781350) as the dominant hydrogen producing microorganism in their study.

Microorganisms can act as adsorbents for the removal and recovery of heavy metals by biosorption. Although biosorption is a relatively new process, it offers a series of advantages including low cost, high metal-removal efficiency, reduced chemical use, reuse potential of biomaterials and nutrients, and possibility of metal recovery. Besides, microorganisms can also facilitate precipitation by catalyzing oxidative and reductive processes that lead to the precipitation of contaminants, such as iron, uranium, and chromium. Some microorganisms can also liberate phosphate and enhance metal phosphate precipitation.

Manikandan et al. examined the ameliorative effect of phosphorous supplementation with Cd-induced toxicity on the morphological and biochemical parameters in Vetiver grass [Chrysopogon zizanioides (L.) Roberty]. It was observed that plant growth was significantly affected by the presence of $50\mathrm{mg}\mathrm{Cd}/\mathrm{L}$, whereas the plant biomass growth increased by 13% with the addition of phosphorous in the medium. The addition of phosphorous also improved the tolerance of the plant to Cd inducted toxicity by activating multiple defense mechanisms. Dutta et al. studied the biosorption properties of ethylenediaminetetraacetate (EDTA)-treated biomass of Baker’s yeast (Saccharomyces cerevisiae) for the removal of Cd, Pb, and Cu from synthetic wastewater containing a mixture of these metals. The authors observed competitive biosorption and a low biosorption capacity of the yeast biomass for the tested metal ions as compared to those under single-metal conditions. From the kinetic viewpoint, the metals were adsorbed by the biomass within a short contact time indicating that the sorption process is mainly confined to the biomass surface.

Kiran et al. demonstrated the potential of anaerobic biomass from three different anaerobic packed-bed type wastewater treatment systems for heavy-metal removal under sulfate reducing conditions. The biomass from a lab-scale upflow anaerobic packed-bed reactor (UFAR) showed a maximum sulfate reduction efficiency ($>90\%$) within $<96\mathrm{h}$, achieving a maximum COD removal of $>92\%$. The heavy-metal removal efficacy was in the order $\mathrm{Cu}>\mathrm{Fe}>\mathrm{Ni}>\mathrm{Pb}>\mathrm{Cd}>\mathrm{Zn}$. The authors attributed the heavy-metal removal mechanism to the capability of the biomass to reduce sulfate to sulfide, thereby resulting in bioprecipitation of the metals as their sulfide salts.

Martins et al. screened eight Penicillium species (P. brasilianum, P. citrinum, P. funiculosun, P. janczewskii, P. janthinellum, P. mineoluteum, P. pinophilum, and P. sclerotiorum), isolated from Brazilian soil, for their ability to remove a mixture of metals, including Cd, Co, Cu, Li, Pb, and Ni, from aqueous solution. Interestingly, the authors reported selectivity of the Penicillium species toward specific metal ions. For instance, P. brasilianum showed a very high affinity for Pb when it was present in combination with Cu and Li. Bhattacharya et al. reported a case study on biofilm reactors for the removal of Hg from aqueous solution using freeze-dried Bacillus cereus (JUBT1) and showed that the freeze-dried strain renders the same characteristics as that of the native strain that was not subjected to freeze-drying treatment. From the view point of bioreactor operation, 1 h residence time was sufficient for the removal of Hg, at an initial Hg concentration in the range of $0.005–0.1\mathrm{mg}/\mathrm{L}$. Tao et al. investigated the resistance of surface-engineered cells of Saccharomyces cerevisiae toward pH, temperature, and silver. The surface-engineered cells showed the highest adsorption capacity of $2.8\mathrm{mg}/\mathrm{g}$, which was 87.3% higher than the original cells. Furthermore, the engineered cells were able to tolerate an acidic environment ($\mathrm{pH}=3.0$ to 5.0) and high temperature (up to 50°C), yielding relatively stable adsorption performance.

Biologically produced nanoparticles, also known as biogenic nanoparticles, are produced by the microbial reduction of heavy metals present in wastewater. For instance, the source of selenium (Se) oxyanions can be wastewater from petrochemical industry, where microbial reduction is employed as a remediation technology for the removal of Se. The formed biogenic Se nanoparticles are usually colloidal polydisperse particles with negative surface charge and are present in the effluent of the bioreactor. Recent research studies have focused on the recovery of these nanoparticles for further use in the bioremediation of environmental pollutants.

Kayalvizhi et al. proposed a rapid method for the synthesis of nanoparticles using Curculigo orchioides rhizome extracts (CoRE) with high levels of polyphenols, gamma sterol, and n-hexadecanoic acid contents. In this paper, the authors discussed the bioactive components present in the rhizome of C. orchioides and green synthesis of silver nanoparticles using rhizome aqueous extracts (boiled and centrifuged) and ascertained its antibacterial, larvicidal, and anticancer activities. Pandi et al. studied the removal of selenium (Se) from synthetic wastewater using marine Aspergillus terreus in an upflow bioreactor and examined the induced selenium stress on the fungal pellets to produce Se nanoparticles. Nearly 85–87% Se was removed by the fungus in the pH range of 6.0–7.0 and a contact time of 5 days. The formation of Se nanostructures with a 500-nm radius on the fungal cell wall was confirmed using scanning electron microscopy (SEM). Rangabhashiyam et al. prepared a novel nanobiocomposite, hydrous cerium oxide nanoparticles impregnated Enteromorpha sp. (HCONIE), and evaluated its potential to adsorb Cr(VI) from aqueous solutions. The adsorption process followed pseudo-first-order rate kinetics. However, in the presence of other metal ions, a slight decrease in the Cr(VI) adsorption capacity was observed when Ni, Co, and Cl coexisted in the aqueous solution. The authors attributed this decrease in adsorption capacity to the presence of competing coions and other factors such as electrostatic interaction and surface complexation on the surface and pores of the nanobiocomposite.

Yn et al. presented a detailed description of the metal nanoparticles (NPs) produced by various marine microbes in the presence of biomass and cell free extracts (CFE). The authors described a one-step protocol for the biosorption of AuCl ions, bioreduction of Au(III) ions to Au(0) NPs, and the subsequent bioengineering of Au biofilm on the surface of microbes following the bioreduction step. X-ray photon spectrometry (XPS) analysis showed that the microbial consortium synthesized zero-valent Au and Ag NPs from synthetic wastewater whereas microbial isolates could only synthesize AgCl NPs using 1 mM AgCl as the precursor. On the other hand, NPs synthesized in the presence of CFE produced well-dispersed NPs with a narrow particle-size distribution.

The deterioration of sewer quality is a hot topic in the field of urban drainage and the continual accumulation of solids or sediments during storm events is a matter of urgent concern. Sedimentation not only creates hazardous conditions, but it also increases the pollutant load to the receiving treatment systems. The reduction and oxidation of sulfur in sewer networks can occur both chemically and biologically, thus complicating the overall process. To date, the mechanisms related to microorganism mediated reduction and oxidation of sulfur compounds in sewers are still unclear, particularly the microlevel environmental effects of wastewater characteristics on sewer biofilm activities, the interactions between sulfur and other organic and inorganic chemical species, and the fate of the different by-products formed as a result of these reactions. Liu et al. reviewed the sulfur cycle by in situ analysis in the sediment biofilm of a sewer system. According to the authors, improved understanding of the mechanism of sulfur cycles in sediment biofilms will promote the development of an adequate control technology to prevent the production and emission of hydrogen sulfide and other odors in the sewer system. The review focused on the physicochemical and biological characteristics of sediment biofilm, substrate profile, and distribution in sewers, microbial community distribution, and shifting in sediment biofilms, reduction of sulfate and oxidation of sulfide in sewers, in situ techniques for monitoring sediment activities and, finally, model development of the sulfur cycle in sediment biofilm of sewers.

In the paper by Yuan et al., the authors performed a case study to understand the speciation and bioavailability of heavy metals in sediments from a wetland in the Huaihe River basin in China. The scope of this work included the quantification of heavy-metal fractions in sediments, evaluation of the historical pollution process, and assessment of the mobility and potential bioavailability of these metals in the sediments from a riverine wetland located in the tributary of the river basin. The results showed significant positive correlations between the total organic carbon (TOC) and different fractions of metals, suggesting the fact that the heavy metals, especially the reactive fractions, were physically absorbed by organic materials. Besides, based on the Enrichment Factor (EF) and Index of Geoaccumulation (IGEO) calculations, the authors determined a remarkable increase in the mobility and bioavailability of Cd, Zn, Pb, Cu, and Cr.

The mechanisms governing the bioremediation of heavy metals and other pollutants have not yet been completely understood in soil, water, and sediment environments, despite the availability of modern analytical and molecular biology tools. Several studies in the past have performed experiments under both batch and continuous modes using live/dead cells, pretreated dead cells, and immobilized cells in different bioreactor configurations for understanding the removal of metals from complex polluted environments. However, the practical application of some of these processes is still limited due to the absence of more in-depth information on the biochemical pathways, enzymatic reactions, reaction kinetics, and rate-limiting steps, the presence of inhibitory compounds and the lack of expertise to scale up some of the bioprocesses from lab scale to field scale.

Ghosh et al. discussed the role of different microorganisms involved in the removal of metals and metal-containing complex dyes from wastewater, their mechanism of pollutant removal, and the different metal recovery processes reported in the literature by considering the technoeconomic issues that need to be considered for an effective scale-up of the bioprocess. From a practical perspective, the authors recommended the use of animal or plant waste for media cultivation and immobilized cells in bioreactors because of their recycle/reuse ability, which would largely reduce the operational and maintenance costs. Shakya et al. reviewed recent developments pertaining to the use of fungi as a biosorbent for heavy-metal removal and recovery from wastewater. The influence of factors like pH, temperature, biomass loading, contact time, type of fungal species, and the presence of other competing metal ions on fungi-metal interactions was addressed in their review. Anew, the authors also highlighted the role of bioluminescent fungal biosensors to monitor and scrutinize the bioavailability of metals, nutrients as well as other toxic organic pollutants present in polluted environments.

The guest editors of this special issue strongly believe that the papers presented in this issue will be of very high interest to all researchers and practitioners in the field worldwide. The guest editors thank all the authors for submitting their high-quality work for publication in this special issue. The reviewers of these papers are greatly acknowledged for their timely and voluntary services, without which this special issue would not have been possible. Thanks to Prof. Raymond A. Ferrara and Prof. Dionysios D. Dionysiou for accepting our proposal to have this special issue in the Journal of Environmental Engineering and the entire editorial and production team at the American Society of Civil Engineers for their excellent support. The guest editors thank their respective organizations—UNESCO-IHE (Delft, Netherlands) and IIT Guwahati (Assam, India)—for their kind support in making this special issue a success.