**3. Treatment of alkaline leachates**

Many of the processes used to treat AMD described in the previous sections are similarly used to treat alkaline leachates generated at disposal sites of industrial by-products. Neutralization or partial neutralization of alkaline leachates prior to discharge to a pH of approximately 8.5–9 using acids is a commonly used treatment process. Recent attempts have been made to examine passive neutralization techniques such as passive in-gassing of atmospheric of CO2 [30], seawater and industrial waste brines [31]. Apart from proximity to an available supply, one of the main disadvantages of seawater neutralization is the high amount of seawater needed and so use of concentrated brines have also been investigated, mostly limited to laboratory studies [31]. Use of CO2 may ultimately provide an attractive neutralization option for alkaline leachates, particularly if CO2 emissions from the processing plant or from adjacent industries could be recycled, thereby reducing their carbon footprints. However, while passive remediation using CO2 has been examined at a laboratory scale [30], further investigation is required at a pilot scale to evaluate issues such as pH rebound. To overcome this, a hybrid process combining CO2 and brine treatments might be considered where the CO2 converts hydroxides to insoluble carbonates and bicarbonates leading to a more stable leachate with a pH < 8.5 [31].

Biosorption of alkaline leachates has also been investigated with varying degrees of success. There are many factors which influence the metal adsorption capacity of biosorbents, including biosorbent characteristics such as pore volume and specific surface area, ionic strength, contact time, adsorbent dosage, solution temperature, initial metal concentration, and solution pH. Solution pH is one of the more important factors, particularly where electrostatic interactions play a key role in the adsorption process such as metal removal from alkaline leachates. The pH at which the net surface charge of the adsorbent is zero is termed the zero point charge (pHzpc). When the pH is less than the pHzpc, the adsorbent surface becomes positively charged and therefore has a high affinity towards negatively charged or anionic metal species. Conversely, when the pH is greater than pHzpc, the surface becomes negatively charged and has an affinity towards positively charged or cationic metal species. In general, optimum metal adsorption is more common at acidic rather than basic pH values, given that alkali metals form cations, which are not attracted to protonated functional groups on the adsorbent surfaces. For example, in a batch study to investigate the efficacy of red mud modified sawdust biochar to adsorb V from aqueous solution, the authors reported a maximum uptake of 16.5 mg g−1 at a solution pH in the range of 3.5–5.5. They attributed the higher uptake levels at low pH to the positively charged adsorbent surface, which enhanced its binding capacity for anionic metal species in solution [32]. However if the pHzpc of the adsorbent is relatively high, then this may promote the removal of heavy metals in alkaline solution. In a batch study to measure the uptake of Cu(II) and Cd(II) from aqueous solutions using ferromanganese binary oxide-biochar composites with a pHzpc = 9.2, the authors reported maximum uptakes of 64.9 and 101 mg g−1 respectively, which increased as the pH increased from 3 to 6. Other adsorption studies at high pH values have also been reported (**Table 3**).

The use of CW's to treat alkaline leachates has also proved successful and as for AMD, the treatment processes rely on metal precipitation, sedimentation, sorption, biological activity and vegetation. The ability of CW's to buffer pH is a key treatment mechanism and recent studies have shown that CW's are effective in quickly reducing *Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial… DOI: http://dx.doi.org/10.5772/intechopen.103889*


**Table 3.**

*Selection of metal adsorption studies at high pH values.*

pH from pH ≈ 11–13 to pH ≈ 7–10 with associated reductions in concentrations of Ca, Al, Ba, Cr, Ga, Ni, Zn and V; however a pH < 10 was needed for effective V reduction [35, 36]. Similar to AMD treatment, the use of CW's to treat alkaline leachates is an attractive long term passive treatment option; however while their longevity in the short term (ca 5 years) has shown to be effective, there is a lack of data to assess their long term performance, particularly with regard to metal concentrations and metal forms in the sediments [7]. While there is no evidence to date of metal accumulation in the CW vegetation treating alkaline leachates [35, 36], the long-term risks of metal saturation in the sediment and metal resolubilisation due to pH changes with concomitant increase in treated effluent concentrations needs to be assessed over a sustained period and under varying operating conditions to establish the long term viability of CW's.

## **4. Resource recovery from acidic and alkaline solid wastes and leachates**

Large quantities of acidic and alkaline wastes are disposed of in storage facilities generating large quantities of metal rich leachates, which are potentially valuable but also toxic to the environment. Several attempts have been made to recycle these wastes but currently waste production far outstrips demand for their reuse. Potential uses for these waste materials are discussed below, many of which focus on the construction industry as an outlet.

### **4.1 Recycling of mine wastes**

Mine tailings have been trialed for use as additives for the production of cement, building bricks and road construction materials, mainly aggregates and asphalt. The use of electric arc furnace steel slag and copper mine tailings were investigated as suitable substitutes for granite aggregates in road asphalt mixtures and were found to improve their performance when compared with conventional aggregates [37]. Similarly, magnetite tailings were used as a substitute for limestone aggregate in asphalt mixtures and were found to improve their high temperature properties and slightly decrease their splitting strength at low temperature [38]. The impact of copper mine tailings blended with cement mortars was also investigated and results indicated that their addition enhanced the mechanical strength of the mortar as well as increasing their resistance to chloride and acid attack [39]. Other studies however have noted that use of mine tailings for cement production involves increased energy consumption, increased dust generation and large emissions of CO2, and have instead advocated the use of geopolymerization as a sustainable process. Geopolymerization chemically

binds natural occurring silico-aluminates to form a stable material (geopolymer) with an amorphous polymeric structure. It has an advantage over other recycling processes in that it reduces the leaching potential of the waste, locking about 90% of the metal content into the geopolymeric matrix [39]. This process therefore requires mine tailings, which are rich in silicon (Si) and Al, both of which are essential materials for geopolymerization. Although an emerging technology, potential applications and properties of geopolymers are their high mechanical strength, good durability, good fire resistance (up to 1000–1200°C without loss of function) and are fast setting making them suitable for use as construction materials such as geopolymer concrete. They also have low energy consumption and generate low waste gas during manufacture with associated reduced CO2 emissions (≈80% reduction) when compared with production of Ordinary Portland Cement [39]. However, as with many innovative emerging technologies, the long-term release of toxic metals from geopolymers requires further research in relation to its eventual leachability during weathering.

Sludge produced from AMD has also been used as an adsorbent for treatment of agricultural waste. In one such column study, the authors reported that AMD sludge was potentially an effective low cost adsorbent for the removal of phosphate from dairy wastewater [40]. It is worth remembering however, that pollution swapping must be considered when applying new recycling technologies and care should be taken not to increase one pollutant as a result of introducing a measure to reduce a different pollutant. For example while one pollutant (phosphorus) might be reduced, metals associated with the adsorbent may be released in the long term and this need to be assessed as part of a life cycle assessment (LCA) to evaluate the overall environmental impacts.

Metal recovery from AMD (as opposed to acid mine waste) remains a technically difficult process where selective precipitation remains the most common treatment method. Separation of a particular metal from a matrix of other metals in solution is a difficult one and typically involves an integrated process, particularly at low metal concentrations. The viability of such processes depends on the economic value of the target metal to be recovered as well as its relative concentration. Treatment technologies such as microbial fuel cells, biological sulfide precipitation, sulfate reducing bacteria, membrane separation and adsorption remain the most promising recovery methods in conjunction with coagulation and precipitation processes and the use of aeration and oxidation to improve efficiencies. While these technologies present opportunities for metal recovery, there are also challenges with their development, not least their economic and environmental viability. It is worth noting that, in addition to trace metals, water, rare earth metals and sulfuric acid are also valuable resources contained within AMD discharges.

### **4.2 Recycling of alkaline wastes**

### *4.2.1 Municipal solid waste incineration byproducts*

The final residue after MSW incineration is generally <10% of the original volume and < 30% of the original mass and typically comprises (i) bottom ash, made up of non-combustible organic matter and inert materials such as glass, ceramics and metals; (ii) grate siftings, fine materials which pass through the grate and are collected at the base of the combustion chamber; (iii) boiler and economizer ash, coarse particulate matter contained in flue gases and usually collected at the heat recovery section; (iv) fly ash, fine particulate matter downstream of the heat recovery section and (v) air pollution control (APC) residues in the form of particulate

### *Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial… DOI: http://dx.doi.org/10.5772/intechopen.103889*

material captured prior to gas emissions to the atmosphere. The amounts of residue generated depend on the composition of the MSW, and the type and efficiency of the incineration process; however typical quantities, expressed as a percentage of the original waste mass on a wet basis, are 20–30% for bottom ash and grate siftings, 10% for boiler and economizer ash, 1–3% for fly ash, and 2–5% for APC residues.

Treatment and disposal of incineration ash residue varies considerably between countries. In China for example most of the bottom ash, an estimated 11 Mt. annum−1, is disposed directly to landfill without pretreatment while in France, Denmark and the Netherlands 80, >90 and 100% respectively of bottom ash is used for road construction and embankments with the balance sent to landfill [41]. Similarly, in countries such as Belgium, Germany, Norway, Spain and Sweden reuse of bottom ash is incentivized resulting in ongoing efforts to establish new outlets for its reuse [42]; however, in the USA almost all incinerator ash is sent to landfill [43].

Typically, scrap iron and other metals are recovered from bottom ash before being landfilled or reused in the manufacture of different types of construction materials. Such applications may be commercially viable but have limitations, for example if ashes with high salt concentrations are used in the manufacture of cement, this may lead to accelerated corrosion of steel reinforcement. Bottom ash is most frequently used as a road granular sub-base material but is more susceptible to leaching in unbound aggregate than in cement bound or ceramic materials which lock in the heavy metals thereby restricting their leachability [44].

Recycling of fly ash, boiler and economizer ash, and APC residues on the other hand is at very low levels with almost all of these being landfilled [41], although other treatments such as thermal processing (melting technology) are also used [45]. The main reason for high disposal rates of fly ash to landfill is that fly ash typically contains high concentrations of heavy metals (for example Zn concentrations can be as high as 60,000 mg kg−1), salts and organic micro pollutants due to their volatization and subsequent condensation during the incineration process. For this reason, fly ashes have a low reuse potential, for example in the cement industry, compared to other secondary raw materials. They are therefore classified as hazardous waste in many countries, which is disposed to either hazardous waste landfills or cement stabilized prior to disposal to non-hazardous waste landfill sites. Similar to bottom ash, one of the main environmental difficulties with recycling fly ash is its leaching potential and consequently there is an emphasis on improving its quality so that it can be used in more sustainable applications.

### *4.2.2 Industrial waste incineration byproducts*

Coal fired power plants are one of the main global energy sources and currently contribute over 40% of power generation. Consequently coal combustion ashes are a major source of economic and environmental concern with >750 Mt. coal ash generated annually and < 50% reused with the remainder generally disposed to landfill or impounded. In Germany for example, approximately 10 million tonnes of stabilized ash is produced annually from lignite combustion power stations [46]. Globally, approximately 25% of coal fly ash is reused with the remainder disposed as waste to landfill. Coal fly ash is an alkaline residue with a variety of trace metals including barium (Ba), boron (B), cadmium (Cd), Co, Cu, Cr, mercury (Hg), Ni, Pb, Mn, tin (Sn), strontium (Sr) V and Zn [47]. While some of these metals are attenuated, at least in the short term, by the alkalinity of the fly ash, other oxyanionic species are released with consequent adverse environmental impacts. Recent research has focused on their

recovery as critical and rare earth elements. Similar efforts are being made with regard to other industries including the steel industry and hazardous waste incinerators. In Sweden for example, trials have indicated recovery of >95% Fe and Mn, and 40% Zn from bottom ash from a crushed alkaline battery incinerator. These represented the metals with the highest concentrations in the bottom ash (Fe, 143,800 mg kg−1; Mn, 154,600 mg kg−1 and Zn, 65,810 mg kg−1). In a separate hazardous waste incinerator trial, valuable metals such as Ni, Sb, Mo, Zn, Cr, and Cu were recovered along with significant quantities of soluble salts, which can be subsequently used as deicing agents on motorways [unpublished data].

### *4.2.3 Bauxite residue reuse*

Bauxite residue is characterized by extreme alkalinity (pH ≈ 10.5–13.5), its red color due to high Fe3O3 content (≈10–50%) and its similarity to clay in terms of its mechanical and physical properties. It also has high concentrations of aluminum oxide (Al2O3, < ≈10–20%). The extreme alkalinity and leachability potential are the main barriers to its reusability, which is considered to be mainly in geotechnical engineering applications [48]. However, given its low strength, poor hydraulic conductivity and relatively poor compactability, additives may be required to render it suitable for many applications including as a road construction material [48]. Changes (reductions) in pH over time may also contribute to long-term leaching, resulting in potentially toxic metals being released to the environment and further research is needed to assess for example the application of pozzolanic materials as a low cost stabilization method.

The application of bauxite residue as an additive to masonry materials has also been investigated. For example, in a study to evaluate the use of bauxite residue co-mixed with agricultural residues as an additive to replace clay in the production of ceramic bricks, the authors concluded that samples produced with an additive of 10% hazelnut shells and 30% bauxite residue resulted in acceptable thermal conductivity and compressive strength values (0.45 W/mK and 9 MPa respectively). Importantly the authors reported that leaching toxicity values were within acceptable Environmental Protection Agency limits [49].

Similar to AMD, metal recovery from alkaline leachates is a technically difficult process, which tends to rely on selective metal precipitation. In a study to investigate V adsorption from aqueous solution by potassium hydroxide (KOH) modified seaweed hydrochar, the authors assessed the reusability of the adsorbent and found that while the adsorption levels remained consistent over three cycles, the physical condition of the adsorbent was the limiting factor in terms of recycling [50]. Thus, further investigation of low cost organic biosorbents in terms of mechanical and physical parameters such as particle size, hydraulic conductivity and porosity in a continuous flow system, as well as life cycle assessment are needed to develop the technology to a higher level.

### **5. Conclusions**

Production of acidic and alkaline mine wastes is expected to continue into the future, with ever increasing amounts of acid mine drainage and alkali leachates being

### *Sustainable Treatment of Acidic and Alkaline Leachates from Mining and Industrial… DOI: http://dx.doi.org/10.5772/intechopen.103889*

generated over a multi decade timescale. The detrimental impacts of these leachates on the aquatic environment is evident with in excess of 18,000 km of streams polluted or projected to be polluted from the coal mining industry alone in north America. Global treatment and remediation costs for existing and abandoned mines is significant, estimated in the range \$32–72 billion, while the remediation costs of treating AMD at abandoned mine sites is estimated to be higher than at operational sites. The difficulty with treatment of acidic or alkaline leachates is that metals tend to exist in low concentrations and in complex matrices. Current active treatment processes such as chemical precipitation, ion exchange, membrane filtration, and coagulation and flocculation processes require ongoing chemical and maintenance inputs, energy usage and treatment of metal rich sludges. To overcome these disadvantages, there has been a recent emphasis on developing passive and sustainable treatment solutions, which do not require continuous chemical and energy inputs. Passive treatment methods such as neutralization, adsorption/biosorption and constructed wetlands are considered to be some of the more promising techniques; however they are not yet fully proven and their ability to effectively treat AMD and alkaline leachates in the long-term is largely unknown. These and other technologies, including hybrid solutions, require further research for long term and sustainable treatment.

Many attempts have been made to reuse disposed acidic and alkaline wastes; however their production far outstrips their demand for reuse at present. The construction industry is a key outlet for mine and industrial waste reuse in products such as aggregates for road construction, cement manufacture and masonry materials. For many applications however, the long term performance of recycled wastes is uncertain and in many cases their use may require increased energy inputs resulting in higher CO2 emissions when compared with traditional materials. To overcome these disadvantages, recent developments of geopolymer based products, formed from Si and Al rich mine tailings, are regarded as a promising emerging technology. Geopolymer based products have good mechanical and durability properties, which potentially make them suitable for use in a wide variety of construction materials. Additionally, the geopolymerization process binds in metals thus reducing their potential for long-term leaching.

With the identification of some metals as 'critical' for modern technology and their availability unpredictable, there has been a recent interest in examining routes to recover such valuable and sometimes scarce metals from mine and mineral processed waste. Metal recovery from the large volumes of leachates generated from acidic and alkaline wastes have had limited success, predominantly due to the complex nature of the metals which tend to exist in low concentrations. However, recent studies have reported efforts to enhance the metal adsorption properties of abundantly available biowaste materials which are sourced from other industries (e.g. agriculture/ aquaculture industries) to facilitate selected metal recovery from mine and industrial leachates. This type of research fits well with the circular economy model of production and consumption, and reinforces the idea of using biowaste from one industry as a raw material to recover valuable resources from another. Although promising, these and other research developments need further technological and life cycle assessments to enhance their technology readiness levels, prior to implementation at an industrial scale.
