Sustainable Heavy Metal Remediation

Metal contamination in the environment is one of the persistent global environmental problems and their adverse health effects have been well documented. Heavy metals can be found in various forms, including fine particles, liquid and gas. On the contrary, metal resource depletion also has accelerated dramatically during the twentieth century owing to advances in industrial engineering and sciences, which require large amounts of raw materials. Therefore, researchers have started to focus on developing technologies which can remove metals from the environment and recover them to reuse as material.

In this chapter, the source and characteristics of several metal contaminated waste streams, recent developments and the technical feasibility of applying physico-chemical and biological technologies/processes to the treatment of wastewater polluted with heavy metals are reviewed. The source of metal pollution will be demonstrated from excavation to end of life product (cradle to grave), while particular technologies such as adsorption, electrocoagulation, biological sulphide precipitation and phytoremediation will be focused on as solutions for heavy metal contamination of the environment.

Many industrial activities result in heavy metal dispersion in the environment worldwide. Heavy metals are persistent contaminants, which get into contact with living organisms and humans creating serious environmental disorders. Metals are commonly removed from wastewaters by means of physical-chemical processes, but often microbes are also enrolled to control metal fate. When microorganisms are used as biosorbents for metal entrapment, a process called “biosorption” occurs. Biosorption efficiency is significantly influenced by many parameters such as environmental factors, the sorbing material and the metal species to be removed, and highly depends on whether microbial cultures are alive or dead. Moreover, the presence of biofilm agglomerates is of major importance for metal uptake onto extracellular polymeric substances. In this chapter, the effect of the above mentioned variables on biosorption performance was reviewed. Among the environmental factors, pH rules metal mobility and speciation. Temperature has a lower influence with an optimal value ranging between 20 and 35 °C. The co-presence of more metals usually decreases the biosorption efficiency of each single metal. Biosorption efficiency can be enhanced by using living microorganisms due to the interaction with active functional groups and the occurrence of transport phenomena into the cells. The existing mathematical modeling approaches used for heavy metal biosorption were overviewed. Several isotherms, obtained in batch conditions, are available for modeling biosorption equilibria and kinetics. In continuous systems, most of the models are used to predict the breakthrough curves. However, the modeling of complex continuous-flow reactors requires further research efforts for better incorporating the effect of the operating parameters and hydrodynamics.

Heavy metal contamination of groundwater is a worldwide problem. Landfill leachate and acid mine drainage are possible sources for groundwater contamination by heavy metals. Heavy metals from groundwater can enter the food chain through bio-accumulation and bio-magnification, posing a threat to all forms of life. A permeable reactive barrier is one of the technologies employed for remediation of heavy metal contaminated groundwater. The concept of a permeable reactive barrier involves the emplacement of a permeable barrier containing reactive materials across the flow path of the heavy metal contaminated groundwater to intercept and treat the heavy metals as the plume flows through it under the influence of the natural hydraulic gradient. Site selection and selection of reactive media, as well as construction and operation, are some of the challenges faced in the application of permeable reactive barriers. A variety of inorganic and organic reactive media are employed in a permeable reactive barrier to remove the heavy metals. Heavy metal removal is accomplished through processes such as adsorption, precipitation and biodegradation. In this chapter, various aspects of treating heavy metal groundwater contamination using the permeable reactive barrier technology have been reviewed. The major topics include: (1) causes of heavy metal contamination in groundwater, (2) types of reactive media used in a permeable reactive barrier, (3) criteria for selection of reactive media, (4) mechanisms for removal of heavy metals by reactive media, and (5) comparison of performance of various reactive media.

Precipitation is the process of solid formation from solution by means of a reaction. It is most frequently used in the removal and recovery of metals from solution. In scientific terms, precipitation is affected by a chemical reaction that forms a salt whose solubility in solution is exceeded. The thermodynamic driving force causing precipitation is called supersaturation. Definitions of supersaturation are not consistent in the literature, and a variety of equations are used for the calculation of supersaturation. The major mechanisms comprising precipitation are nucleation, growth and agglomeration. High supersaturation levels favour nucleation, whilst lower levels favour crystal growth. Agglomeration occurs in the presence of large numbers of particles, in a supersaturated environment.

Precipitation is commonly used for metal removal from wastewaters, but is not yet commonly used for metal recovery from wastewaters. Metal hydroxide precipitation is the most commonly used method, although metal sulphide precipitation has many advantages. Other methods of metal removal can be in the form of sulphate (e.g. CaSO₄.2H₂O) or fluoride (e.g. CaF) salts.

Crystalliser design for water treatment ranges in complexity from the simplest pipe reactor to the more sophisticated fluidised bed reactor, which is an extremely effective design for metal removal and recovery.

In summary, when using precipitation as an extremely effective metal removal and recovery method, careful attention must be paid to designing precipitation systems that are able to produce precipitates with desirable separation characteristics.

Sanitary landfills are the most widely used method of solid waste disposal around the world. Landfill leachate (LL) is recognized as one of the most critical issues for landfill operators. Landfill leachate may contain large amounts of organic matter (biodegradable, and refractory to biodegradation), as well as ammonia-nitrogen, heavy metals (HM) and chlorinated organic and inorganic salts. Various landfill leachate treatment technologies have been broadly used, including biological processes (aerobic, anaerobic and anoxic) and physicochemical processes (oxidation, precipitation, coagulation/flocculation, ozonation, activated carbon adsorption, electrochemical oxidation, Fenton process, membrane filtration). Constructed wetlands are classified among the biological methods that use phytoremediation for polluted liquid treatment. They are defined as engineered systems that use natural processes (vegetation, soils and microorganisms) to remove, transform and degrade pollutants from wastewater, creating an efficient synergic effect. The effectiveness of constructed wetlands for landfill leachate treatment has been extensively demonstrated and its full-scale implementation is rising among regions given its adaptability and capacity to efficiently treat landfill leachate.

Although metal bearing wastes are toxic, they possess economic value and hence need attention towards remediation/recovery. Various physical and chemical methods are being practiced for treating metal laden wastewaters, but are limited owing to the problems associated with maintenance and operational costs. Biological methods that use microbes as catalyst are cost effective and easy to operate, but only a little progress has been made in terms of recovery than the treatment. Recently, there is a shift in focus from bioremediation of metal wastes towards the recovery of valuable metals which are scanty. In this context, bioelectrochemical systems (BES) have emerged as a potential technological platform for recovery of metal ions from metallurgical waste (end-of-life products), process streams and wastewaters. In bioelectrochemical systems, microbial oxidation of organic substrate at the anode is coupled to abiotic or biotic reduction of metal ions at the cathode. With this perspective, this chapter gives an insight on the redox mechanisms of bacteria towards metal recovery along with the influence of in situ and ex situ potentials in bioelectrochemical systems. The exo-electron transport mechanism in bacteria for metal reduction and speciation is also discussed. Besides, the chapter also provides an overview on the metal speciation in bioelectrochemical systems along with electrochemical, physical and chemical methods for metal removal and recovery from wastewaters. Emerging metal recovery concepts based on bioelectrochemical systems are also presented in detail.

Heavy metals are toxic, carcinogenic and unlike organic contaminants are not biodegradable, and thus accumulate in organisms. Approximately 60% of the polluted areas in the world, suffer from the harmful effects of metals including Cd, Ni, Cu, Pb, Zn, Hg and Co. Mining, fertilizer, tanneries, paper, batteries and electroplating industries are the main sources of heavy metal containing waters. For example, in China, the annual amount of heavy metal containing electroplating industry wastewater has exceeded 4 billion tons. Up to 1000 mg/kg heavy metal concentration in sediments has been reported due to repeated discharges. We reviewed the sources of heavy metal containing water and metal precipitation techniques including metal sulfide, hydroxide, ferrihydrite, geothite, jarosite as well as schwertmannite precipitation. Metal sulfide precipitation relies on the biological generation of H₂S and near complete metal removal is possible with both organic (i.e. ethanol) and inorganic (i.e. hydrogen) electron donors. The utilization of soluble electron donors provides high rate and dense metal precipitates with metal recovery of over 80% (usually 100%). Additionally, metals can be recovered separately as various metal sulfides by adjusting pH. Biological oxidation/reduction processes facilitate the formation of insoluble metal precipitates for uranium (U⁶⁺ to U⁴⁺); chromium (Cr⁶⁺ to Cr³⁺) or iron (Fe²⁺ to Fe³⁺). The major points extracted from the study are: (1) metal sulfide precipitation is fast, results in low residual metal concentrations and allows for selective recovery of various metals with a wide variety of different reactor configurations, (2) high rate biological metal recovery is possible with cultures which use metals as electron acceptors which eliminates the drawbacks such as chemical costs and huge sludge volume production in chemical reduction, (3) animal manure, leaf mulch, sawdust, wood chips, sewage sludge, cellulose could be used in passive treatment systems and therefore operational costs could be optimized, (4) some heavy metals can be precipitated through biological oxidation (i.e. Fe²⁺ to Fe³⁺) and (5) possible iron precipitates include hematite (Fe₂O₃); geothite (FeOOH); ferric hydroxide Fe(OH)₃; jarosite Fe₃(SO₄)₂(OH)₆; schwertmannite Fe₁₆O₁₆(SO₄)₂(OH)12.n(H₂O) and scorodite (FeAsO₄.2H₂O).

The solubilisation of metals and metalloids is catalysed by a variety of microorganisms in natural and engineered environments. Biosolubilisation has a number of undesired implications, such as the generation of acid mine drainage and the formation of acid sulfate soils, which have harmful environmental impacts. Biosolubilisation also contributes to the corrosion of man-made structures causing significant economic losses. On the other hand biosolubilisation has been harnessed by the mining industry to recover valuable metals and uranium from low-grade ores and concentrates in large scale. This allows the utilisation of ores the processing of which would not be economically feasible through traditional mining methods. Biosolubilisation holds also potential for the recovery of resources from waste and clean-up of metal contaminated environments. This chapter reviews the role that microorganisms have in the solubilisation of various metals and metalloids, the mechanisms through which biosolubilisation occurs and microbial groups mediating the solubilisation. The environmental implications and industrial applications of biosolubilisation are also discussed. Microorganisms can catalyse biosolubilisation through oxidative and reductive dissolution, mediated by the oxidation and reduction of ferrous and ferric iron, respectively. Moreover, biosolubilisation can be achieved through the production of biogenic acids, alkali and ligands, such as cyanide, thiosulfate, organic acids and iodide. Mechanisms contributing to microbially influenced corrosion of metallic iron and steel include differential aeration cells, galvanic cells, attack by microbial oxidants, acids, sulfides and other metabolites, cathodic depolarisation and direct microbial extraction of electrons from steel. A wide range of microorganisms are able to facilitate solubilisation reactions, including bacteria, archaea and eukaryotes. Bioleaching has been explored for recovering metals from e.g. a variety of sulfide ores, metallurgical waste, electronic scrap, sludge from municipal and industrial wastewater treatment, municipal solid waste incineration fly ash and contaminated sites. Large-scale biosolubilisation has been mainly used for copper-, cobalt-, nickel-, zinc-, uranium- and gold-containing sulfidic ores through oxidative bioleaching, whereas reductive bioleaching is yet to be implemented at industrial scale.