Issues Concerning Manufacture and Recycling of Lead

Citation: Baˇca, P.; Vanýsek, P. Issues Concerning Manufacture and Recycling of Lead. Energies 2023,16, 4468. https://doi.org/10.3390/ en16114468 Academic Editor: Alessandro Del Nevo Received: 30 April 2023 Revised: 23 May 2023 Accepted: 29 May 2023 Published: 1 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Issues Concerning Manufacture and Recycling of Lead Petr Baˇca * and Petr Vanýsek The Faculty of Electrical Engineering and Communications, Brno University of Technology, Technická3058/10, 616 00 Brno, Czech Republic; [email protected] *Correspondence: [email protected]; Tel.: +420-54114-6188 Abstract: This article gives an overview of historical and present uses of lead against the backdrop of gradual realization that lead is an environmental hazard. In this paper the lead in the lead-acid batteries is investigated from the view of its present use. This use continues because there does not exist economical and practical alternative to lead for this purpose. In fact the use is still steadily increasing. This may not be a concern as it has been demonstrated that in countries with strong economies, recycling of lead from the batteries can near 100%. Here, we take a look at reality, by comparing select countries on both sides of the economic spectrum. In poorer countries, recycling suffers more on the safe and clean side of the process. Historical uses of lead are also reviewed, as well as a new approach of using lead compounds in soluble lead flow batteries. Keywords: lead metal; lead-acid storage batteries; lead recycling; banned use of lead; environment 1. Introduction The steady stream of discoveries and technical inventions has led to a steady progress in facilitating human activities, improving human lot and identifying and accepting new possibilities. However, in complex linkages, these new opportunities may also have unwanted, undesirable and sometimes unforeseen consequences. An example is the impact on interpersonal relationships or disruption of environment. Often these consequences are gradual and become apparent only after many years. At present, many materials that were considered harmless at one time are now blacklisted, and their use is severely restricted or directly prohibited. In terms of obtaining energy, it is already a well-established case that burning fossil fuels has had negative impact, although historically the positive impact of available energy on advancement of humankind cannot be disputed. Issues of generating alternative energy are routinely addressed; here, we will focus on another issue concerning energy potentially affecting the environment, namely the use of lead-acid batteries as the means of energy storage. Lead, in general, has pronounced toxicity [ 1 ]. At low levels of exposure to lead and its compounds, the major concern is the neurological effect which is more pronounced in developing organisms. The presence of lead in the human body damages the nervous system through two mechanisms: morphological and pharmacological [ 2 ]. The morphological mechanism alters the development of the nervous system, in particular, starting from the prenatal period through the childhood [ 3 ]. Pharmacological effects result from the chemical similarity of lead to calcium and to a smaller degree to zinc, and thus incorrectly trigger processes reliant on calmodulin [ 4 ], a protein that regulates the uptake of calcium ion. Exposure to lead and its compounds, either chronic or acute, can induce significant neuropsychological deficits, including negative impacts on intelligence, memory, comprehension and reading [3]. While in the past most of the environmental contamination by lead was airborne, this component has been mainly successfully curtailed. However, water contamination by lead still remains a concerning issue and a monitored parameter of the environmental health. Based on the WHO guidelines, the USA decided to propose a limit value of 15 µg/L Energies 2023,16, 4468. https://doi.org/10.3390/en16114468 https://www.mdpi.com/journal/energies Energies 2023,16, 4468 2 of 20 in drinking water, taking into account the reduction of other sources of lead [ 5 ]. The World Health Organization (WHO) proposal was also integrated in the new EU Drinking Water Directive 98/83/EC (3 November 1998), where the limit of 10 µ g/L was set for implementation on 25 December 2013 [5]. In 2021, a new EU drinking-water directive lowered the limit further to 5 µ g/L, which must be met by 12 January 2036 at the latest [ 6 ]. This is in line with legislation in several other countries. The value is aspirational. The parametric value for lead until that date is 10 µ g/L. However, caution is needed when comparing a real sample with the mandated limits, because interpretation will be influenced by the sampling regime, which may or may not be specified in the regulation. The capability to verify the set standards is tied to the availability of analytical instruments with the appropriate detection limit. Exposure to lead is typically determined through measuring the lead contents in the blood [ 7 ]. The most common instrumental methods of determination of lead concentrations in aqueous samples are flame atomic absorption spectrometry (FAAS), anodic stripping voltammetry (ASV) and inductively coupled plasma mass spectrometry (ICP-MS). While FAAS is rapid and requires only basic laboratory skills, its detection limit is around 10 µ g/L, equal to the current WHO guidance and not sufficient for the EU goal of 5 µ g/L. Electrochemical method AVS is certainly more suitable in terms of its detection limit (2–3 µ g/dL) and a portable and affordable but not as accurate instrument with the detection limit of about 3.3 µ g/dL. Graphite furnace atomic absorption spectrometry (GFAAS) has been somewhat of a gold standard for lead determination with a detection limit below <1–2 µ g/dL [ 8 ] and the instrumental detection limit (IDL) being reported, for example, as 0.38 µg/dL [9]. It requires a small sample, and the instrumental cost is acceptable due to the wide availability of the instrumentation used, but this method requires some laboratory expertise. ICP-MS has the best detection limit (~0.1 µ g/dL, but also reported as low as 0.05 µ g/dL [ 10 ]), but the instrument is expensive with high running costs and requires a highly skilled laboratory operator. To justify the expense, a large number of processed samples is required to benefit from the economy of scale [7]. 2. Historical Uses of Lead To understand the sources of environmental lead, one can follow the use of this metal in consumer products. Lead has been mined for more than 6000 years, and both the metal and its compounds have been used throughout the ancient history. The ancient Greeks had mined lead ore in considerable quantities since 650 A.D. [ 11 ], and not only did they produce metal, they also produced lead white pigment. Lead white has excellent opacity and has been used for this property for over 2000 years [ 12 ], all the way until the mid20th century, when its toxicity was finally universally recognized, and suitable substitutes became available [ 13 ]. The Romans, who mined lead ore in Spain and Britain, used lead for water pipes, coffin lining, storage cans and for adulteration of silver coins [ 14 ]. In the Middle Ages, lead was used in glazing ceramics [ 15 ], making bullets and type casting [ 16 ]. In the first half of the 20th century, a large amount of lead was used to produce tetraethyl lead (Pb(CH 3 CH 2 ) 4 , Pb(C 2 H 5 ) 4 ) as a gasoline additive for internal combustion engines [ 17 ]. Lead white was the most important white paint in history [ 12 ]. It is a basic lead carbonate of general formula PbCO 3· Pb(OH) 2 . Because of its high covering power, it has been widely used as a pigment in interiors. However, as is the case with wall paints, eventually it starts to peel. Then, it takes the form of dust of peeled flakes and as a compound find its way into the human body. Low-grade poisoning can cause neurological problems, but deaths have also occurred when these flakes were swallowed, especially in children, as unfortunately this substance happens to taste somewhat sweet. Although the toxicity of lead compounds has been known for a long time [ 18 ], and the risk of indoor lead pigments has also been recognized early, it took a long time for these pigments to be banned from general use. In France, Belgium and Austria, it happened in 1909 [ 19 ], in most of Europe, their use continued until 1940, but it was not banned in the USA until Energies 2023,16, 4468 3 of 20 1976 [ 20 ]. At present, it is possible to obtain it as a pigment only for oil painting restoration purposes. Nevertheless, lead-based paint is still produced in limited amounts and in some communities presents major public health concern [21]. Lead tetroxide (minium) is also harmful; however, it has been used as a primer for exterior designs (reddish color) of metal constructions, and so the risk of poisoning the general population was much lower. It is also heralded for excellent electrochemical ability to inhibit steel corrosion. In a humid outdoor environment, while reacting with organic material, a lead salt of azelaic (nonanedioic) acid forms, which constitutes a corrosion protective surface [ 22 , 23 ]. Although in limited amounts, minium-based primers can still be obtained for certain purposes, for example, in art supplies stores. Pb(C 2 H 5 ) 4 was in its time a very useful gasoline additive that slowed down the burning of gasoline and increased thus its octane number. The principle had to do with Pb(C2H5)4binding with emerging radicals formed during combustion. However, besides the gasoline quality improvement, the final result was also the emission of lead and its compounds into the atmosphere: Pb(C2H5)4+ 13 O2→8 CO2+ 10 H2O + Pb Tetraethyl lead was first used as a gasoline additive for internal combustion engines in 1921 [ 17 ]. Its use on the mass scale began in the USA when three manufacturing giants, General Motors, DuPont and Standard Oil, joined to have the Ethyl Corporation produce Pb(C 2 H 5 ) 4 as a gasoline additive in 1924 [ 24 ]. That this compound was poisonous was found through accidents very early in its production. However, manufacturers have succeeded in persuading the authorities who were in charge of safety, that the amount of lead in the gasoline, and in particular in the exhaust gas, is so negligible that its presence and environment contamination cannot even be detected. However, with the improvement of analytical methods, the lead contamination of the environment has been found to be growing, and fears that lead could be harmful in these amounts have started to grow. However, the production of Pb(C 2 H 5 ) 4 , the manufacture of which was patented, was lucrative, and so the manufacturers made steady efforts to prevent it use as a gasoline additive from being discontinued. Nevertheless, its use in the US has decreased in 1986 by 91%. The fact that catalytic converters, which were destroyed by leaded gasoline, began to be installed on the car exhausts for clean air reasons eliminated leaded gasoline in automotive industry entirely. Then, in a short time, this additive was replaced by the addition of ethanol, which also modifies the favorable combustion properties of gasoline. The removal of lead from gasoline has often been called the greatest public health success of the past century [25,26]. Lead poisoning is manifested in the neurological spectrum, and both acute and chronic poisonings have been well known. However, at low concentrations from exhaust gases and a concomitant change in the urban environment, it was not easy to point out the harmfulness of lead in the atmosphere from Pb(C 2 H 5 ) 4 . However, a study has been conducted in the US that, based on statistical data, pointed out that the use of Pb(C 2 H 5 ) 4 resulted in an increase in violent crime, or, conversely, twenty-two years after the removal of Pb(C 2 H 5 ) 4 from gasoline, a decrease was noted in recorded criminal violence—at the time when children, who were not exposed to lead, have grown to the age of potentially becoming young adult delinquents [ 27 ] (Figure 1). There is a 21-year delay between the maximum level of lead exposure and the maximum number of committed homicides. The correlation between these offset curves as we determined was 0.7174 on the Pearson test, indicating that these two variables are highly correlated. Nevin [ 27 ] in his original work arrived at the 21-year offset in fact by comparing data correlation for different time offsets and chose the one with the best correlation. While there is certainly strong argument for the relationship, it needs to be pointed out, that such swings observed in Europe, even going to the 19th century, does not show such strong correlation [28]. Energies 2023,16, 4468 4 of 20 Energies 2023, 16, x FOR PEER REVIEW 4 of 21 Figure 1. The dependence of the comparison of the historical lead content curves on the environment and violent crime (murder), inspired by [27]. 3. Present Day Lead Use Even though lead is being excluded from many traditional uses due to its toxicity, it is still used in a number of areas. Over the years, lead pipes, inside tin-plated, have been used for water delivery in indoor plumbing. Although there are no new installations using lead pipes, due to their durability they can still be found in old buildings. Thanks to its high density (11.34 g/cm3), lead is used as a ballast and is commonly found as a weight in fishing tackle, in curtain weights, in weights for diver belts or in automotive wheel for balancing, although lead is now allowed only for the heavy truck wheels. It still has its significant use in cable sheathing, sound deadening of rooms, roof flashing on historical buildings or tank lining for sulfuric acid production to give a few examples of remaining use. Lead weights suspended from a string were also used in the construction industry to detect verticality. From here comes the name plumb line, where plumbum is the Latin for lead. While the term plumb bob is still used, the item has been replaced by a harder and sturdier brass piece. Where lead cannot be replaced by another material is in radiation protection. Therefore, it is still used, for example, to isolate X-ray machines. Lead shot and lead bullets are still the most common projectiles in ammunition, and lead consumption through this use is particularly important in countries with easy access to firearms. However, the greatest lead consumption is in the production of lead-acid batteries. Here, in terms of the chemical charging and discharging process, lead is also irreplaceable. Reversible reactions occur on both electrochemical cell plates according to the following equations: Reaction on the negative plate Pb + HSO4− → PbSO4 + H+ + 2 e− Reaction on the positive plate PbO2 + HSO4− + 3 H+(aq) + 2 e− → PbSO4 + 2 H2O Overall reaction Figure 1. The dependence of the comparison of the historical lead content curves on the environment and violent crime (murder), inspired by [27]. 3. Present Day Lead Use Even though lead is being excluded from many traditional uses due to its toxicity, it is still used in a number of areas. Over the years, lead pipes, inside tin-plated, have been used for water delivery in indoor plumbing. Although there are no new installations using lead pipes, due to their durability they can still be found in old buildings. Thanks to its high density (11.34 g/cm 3 ), lead is used as a ballast and is commonly found as a weight in fishing tackle, in curtain weights, in weights for diver belts or in automotive wheel for balancing, although lead is now allowed only for the heavy truck wheels. It still has its significant use in cable sheathing, sound deadening of rooms, roof flashing on historical buildings or tank lining for sulfuric acid production to give a few examples of remaining use. Lead weights suspended from a string were also used in the construction industry to detect verticality. From here comes the name plumb line, where plumbum is the Latin for lead. While the term plumb bob is still used, the item has been replaced by a harder and sturdier brass piece. Where lead cannot be replaced by another material is in radiation protection. Therefore, it is still used, for example, to isolate X-ray machines. Lead shot and lead bullets are still the most common projectiles in ammunition, and lead consumption through this use is particularly important in countries with easy access to firearms. However, the greatest lead consumption is in the production of lead-acid batteries. Here, in terms of the chemical charging and discharging process, lead is also irreplaceable. Reversible reactions occur on both electrochemical cell plates according to the following equations: Reaction on the negative plate Pb + HSO4 −→PbSO4+ H++2e− Reaction on the positive plate PbO2+ HSO4 −+3H+(aq) + 2 e−→PbSO4+2H2O Overall reaction Pb + PbO2+2H2SO4→2 PbSO4+2H2O Ecell = 2.05 V, ∆G = −393.87 kJ mol−1 Energies 2023,16, 4468 5 of 20 Lead loss and subsequent potential contamination due to lead-acid battery use begins with the initial manufacturing steps of new batteries. One of the essential steps in the manufacturing process is to prepare a grid into which the active mass of both the positive and negative electrode is pasted. In the process, bars of appropriate lead alloy are melted and pumped into molds, producing after some cooling and trimming the desired grids. The trimmed material and off-size grids are re-melted. However, during melting is also generated some dross that needs to be reprocessed or disposed of in a suitable way. One possible method reducing dross formation is blanketing the surface of the molten metal in the melting pots by nitrogen [ 29 ], which is feasible, but requires additional investment and possibly major remodeling of the assembly line. Lead oxide formation from lead in air at higher temperatures, its stoichiometry and subsequent reduction at even higher temperatures is a rather complex scheme. The active material filled into the electrodes can be a lead oxide of several compositions (PbO, Pb 3 O 4 , PbO 2 ) [ 30 ]. To express this composition variability or uncertainty, the material is of the called leady oxide. Commonly, this material is lead monoxide mixed with some percentage of lead particles. Lead oxides are generated by air oxidation of high purity (99.97%) molten lead with temperature up to at 400–425 ◦ C, where the temperature affects the stoichiometry greatly. From the oxide are manufactured pastes for the electrodes, one for positive and one for negative plate. While both contain lead oxide (about 85%) [ 29 ], sulfuric acid and water, they differ in the types of other additives. Afterwards the pastes are rolled into the grids by a mechanized process, which contributes to even coating. Afterwards, the plates are dried. Essential step is curing the plates, involves exposing the pasted positive and negative plates to humid fresh air (above 95% rel. humidity) and a temperature of 30–35 ◦ C for at least 32 h [ 29 ]. In this process, oxygen from the air converts any remaining free lead into lead oxide. At this point, the plates are large and have to be parted, i.e., cut to the desired size for a particular battery assembly. During the assembly, the positive and negative plates are placed in the plastic case of the battery. The assembly also involves intercell welding and melting of lugs. This is the manufacturing process of the so-called dry (uncharged) battery that has to be charged. The battery is filled with sulfuric acid, 1.260 and 1.280 g/mL specific gravity [31]. The charging is provided by specified current for specified time. From the point of view of lead availability, cost, established technology and growing demand for batteries, the lead-acid battery production, compared to other uses of lead, will continue to grow. Figure 2[ 32 , 33 ] shows the history of miscellaneous use of lead, and already in 2014, 90% of refined lead was used for batteries. Energies 2023, 16, x FOR PEER REVIEW 5 of 21 Pb + PbO2 + 2 H2SO4 → 2 PbSO4 + 2 H2O Ecell = 2.05 V, ΔG = −393.87 kJ mol−1 Lead loss and subsequent potential contamination due to lead-acid battery use begins with the initial manufacturing steps of new batteries. One of the essential steps in the manufacturing process is to prepare a grid into which the active mass of both the positive and negative electrode is pasted. In the process, bars of appropriate lead alloy are melted and pumped into molds, producing after some cooling and trimming the desired grids. The trimmed material and off-size grids are re-melted. However, during melting is also generated some dross that needs to be reprocessed or disposed of in a suitable way. One possible method reducing dross formation is blanketing the surface of the molten metal in the melting pots by nitrogen [29], which is feasible, but requires additional investment and possibly major remodeling of the assembly line. Lead oxide formation from lead in air at higher temperatures, its stoichiometry and subsequent reduction at even higher temperatures is a rather complex scheme. The active material filled into the electrodes can be a lead oxide of several compositions (PbO, Pb3O4, PbO2) [30]. To express this composition variability or uncertainty, the material is of the called leady oxide. Commonly, this material is lead monoxide mixed with some percentage of lead particles. Lead oxides are generated by air oxidation of high purity (99.97%) molten lead with temperature up to at 400–425 °C, where the temperature affects the stoichiometry greatly. From the oxide are manufactured pastes for the electrodes, one for positive and one for negative plate. While both contain lead oxide (about 85%) [29], sulfuric acid and water, they differ in the types of other additives. Afterwards the pastes are rolled into the grids by a mechanized process, which contributes to even coating. Afterwards, the plates are dried. Essential step is curing the plates, involves exposing the pasted positive and negative plates to humid fresh air (above 95% rel. humidity) and a temperature of 30–35 °C for at least 32 h [29]. In this process, oxygen from the air converts any remaining free lead into lead oxide. At this point, the plates are large and have to be parted, i.e., cut to the desired size for a particular battery assembly. During the assembly, the positive and negative plates are placed in the plastic case of the battery. The assembly also involves intercell welding and melting of lugs. This is the manufacturing process of the so-called dry (uncharged) battery that has to be charged. The battery is filled with sulfuric acid, 1.260 and 1.280 g/mL specific gravity [31]. The charging is provided by specified current for specified time. From the point of view of lead availability, cost, established technology and growing demand for batteries, the lead-acid battery production, compared to other uses of lead, will continue to grow. Figure 2 [32,33] shows the history of miscellaneous use of lead, and already in 2014, 90% of refined lead was used for batteries. Figure 2. Annual lead consumption according to its use [ 32 , 33 ], image is a part of a composite figure from The Royal Society open access article. Energies 2023,16, 4468 6 of 20 While the world production of lithium-ion batteries surpassed in sales lead-acid batteries around year 2015, the production of the lead-based power sources is still projected to increase rapidly (Figure 3). An article, which critically compares the advantages of Li-ion over lead-acid batteries in motor vehicles [ 34 ], recognizes that even these days, there is significant place for power sources based on lead chemistry. Energies 2023, 16, x FOR PEER REVIEW 6 of 21 Figure 2. Annual lead consumption according to its use [32,33], image is a part of a composite figure from The Royal Society open access article. While the world production of lithium-ion batteries surpassed in sales lead-acid batteries around year 2015, the production of the lead-based power sources is still projected to increase rapidly (Figure 3). An article, which critically compares the advantages of Li-ion over lead-acid batteries in motor vehicles [34], recognizes that even these days, there is significant place for power sources based on lead chemistry. Figure 3. Past and predicted global production of lead-acid and lithium batteries in dollars (compilation from [35,36]). Figure 2 gives a general feeling about the prevalence of lead production. Detailed analysis of available data of lead production by purpose reveals interesting facts about using statistical data. We have analyzed the collection of the U.S. Geological Survey (USGS) yearly reports [37]. While it would be possible to recreate Figure 2 back to year 1920, it would not be very illustrative. Instead, we juxtaposed yearly production of lead used to make lead-acid batteries and lead used to make projectiles for ammunition (Figure 4). The scale magnitude for lead-acid batteries is about ten times larger. Obvious is the trend in demand for lead-acid batteries, as seen in Figure 3, for example, but so is the increase in ammunition use, although not as rapid. A graph that is not shown here displays an increase and then decrease over the studied period for the material used as a composite for lead paint and tetraethyl-lead gasoline additive. Other industrial uses such as for alloys, bearings and cable shielding still remain steady. Unlike scientific measurements, statistical reports rely more on estimates and sometimes on less perfect definition of the data reported. This can be the reason for some of the battery production fluctuations. In early years, there were separate listings for lead for batteries and lead oxides, and the two were added together. In the heyday of lead production, it is not guaranteed that the oxide use was strictly separated. After discontinuation of lead used in paint (ca. 1960), the difference then would be minimal. For the precipitous apparent drop in battery production in 1939–1943 we did not find explanation. This may be due to the reporting of data for domestic use and not any other use for war efforts. Figure 3. Past and predicted global production of lead-acid and lithium batteries in dollars (compilation from [35,36]). Figure 2gives a general feeling about the prevalence of lead production. Detailed analysis of available data of lead production by purpose reveals interesting facts about using statistical data. We have analyzed the collection of the U.S. Geological Survey (USGS) yearly reports [ 37 ]. While it would be possible to recreate Figure 2back to year 1920, it would not be very illustrative. Instead, we juxtaposed yearly production of lead used to make lead-acid batteries and lead used to make projectiles for ammunition (Figure 4). The scale magnitude for lead-acid batteries is about ten times larger. Obvious is the trend in demand for lead-acid batteries, as seen in Figure 3, for example, but so is the increase in ammunition use, although not as rapid. A graph that is not shown here displays an increase and then decrease over the studied period for the material used as a composite for lead paint and tetraethyl-lead gasoline additive. Other industrial uses such as for alloys, bearings and cable shielding still remain steady. Unlike scientific measurements, statistical reports rely more on estimates and sometimes on less perfect definition of the data reported. This can be the reason for some of the battery production fluctuations. In early years, there were separate listings for lead for batteries and lead oxides, and the two were added together. In the heyday of lead production, it is not guaranteed that the oxide use was strictly separated. After discontinuation of lead used in paint (ca. 1960), the difference then would be minimal. For the precipitous apparent drop in battery production in 1939–1943 we did not find explanation. This may be due to the reporting of data for domestic use and not any other use for war efforts. Main lead producers are China, Australia, USA, Mexico, Peru, India and Russia. For example, China produced in 2022 estimated 2,000,000 tons of lead, with the production for the whole world totaling 4,550,000 tons. In all countries, this newly produced lead is mainly used to make batteries. World known reserves are in millions of tons in the following countries: Australia 37, China 12, Russia 6, Peru 5.3, USA 4.6 and India 2.5. The total reserves for all countries amount to 85 million tons [37]. Energies 2023,16, 4468 7 of 20 Energies 2023, 16, x FOR PEER REVIEW 7 of 21 Figure 4. Comparison of lead consumption as lead-acid batteries and lead projectiles in ammunition in the USA, by year. Main lead producers are China, Australia, USA, Mexico, Peru, India and Russia. For example, China produced in 2022 estimated 2,000,000 tons of lead, with the production for the whole world totaling 4,550,000 tons. In all countries, this newly produced lead is mainly used to make batteries. World known reserves are in millions of tons in the following countries: Australia 37, China 12, Russia 6, Peru 5.3, USA 4.6 and India 2.5. The total reserves for all countries amount to 85 million tons [37]. Considering that lead use in a number of applications is being phased out for environmental reasons, it may be valid assumption, that no new uses for lead would emerge. Still, one interesting innovation based on lead, and pertinent to energy and energy storage, is the soluble lead flow battery. Flow battery is a rechargeable type of battery which, unlike a conventional rechargeable system, that has its active material embedded within the battery, has the supply of the active oxidizable and oxidizing material stored in outside tanks. To this end, a flow battery resembles a fuel cell in its function. Flow batteries are developed and marketed as stationary energy storage systems with the advantage that the amount of energy stored can be scaled up simply by providing bigger storage tanks for the reactive material, while the active electrode-membrane assembly can remain unchanged. The soluble lead battery has recently made advances into higher technology readiness level. An economy study [38] assessed feasibility of a 24 V soluble lead flow battery. This was performed from the known data about present capabilities with the perspective of energy storage in photovoltaic applications for a charging hub in Sierra Leone. The authors of the study found out that the electrolyte for the system that would fit into a 1000 L intermediate bulk container could deliver a total of 14 kWh of energy with a power of 3.5 kW. Their study also pointed out the optimal economy operating conditions. For example, a load of 1.64 kW for a four-hour duration was found to be optimal, before diminishing returns began. The simulated efficiency of this system was 59%. The soluble lead redox flow battery (SLFB) builds on the existence of three oxidation states of lead, specifically Pb0, Pb2+ and Pb4+ [26]. Chemistry similarity with a lead-acid battery should be noted, except here, the Pb2+ phase is in soluble form. In the simplest SLFB, Pb2+ ions are dissolved in methanesulfonic acid (MSA), CH3SO3H. This ionically conductive solution is pumped into an undivided electrochemical cell with graphitic inert electrodes and the cell is being charged by applied electric current. At the positive electrode, the Pb2+ ions are oxidized to a solid PbO2, which is deposited on the electrode. Simultaneously, at the negative electrode the Pb2+ ions are reduced to a solid metallic lead Figure 4. Comparison of lead consumption as lead-acid batteries and lead projectiles in ammunition in the USA, by year. Considering that lead use in a number of applications is being phased out for environmental reasons, it may be valid assumption, that no new uses for lead would emerge. Still, one interesting innovation based on lead, and pertinent to energy and energy storage, is the soluble lead flow battery. Flow battery is a rechargeable type of battery which, unlike a conventional rechargeable system, that has its active material embedded within the battery, has the supply of the active oxidizable and oxidizing material stored in outside tanks. To this end, a flow battery resembles a fuel cell in its function. Flow batteries are developed and marketed as stationary energy storage systems with the advantage that the amount of energy stored can be scaled up simply by providing bigger storage tanks for the reactive material, while the active electrode-membrane assembly can remain unchanged. The soluble lead battery has recently made advances into higher technology readiness level. An economy study [ 38 ] assessed feasibility of a 24 V soluble lead flow battery. This was performed from the known data about present capabilities with the perspective of energy storage in photovoltaic applications for a charging hub in Sierra Leone. The authors of the study found out that the electrolyte for the system that would fit into a 1000 L intermediate bulk container could deliver a total of 14 kWh of energy with a power of 3.5 kW. Their study also pointed out the optimal economy operating conditions. For example, a load of 1.64 kW for a four-hour duration was found to be optimal, before diminishing returns began. The simulated efficiency of this system was 59%. The soluble lead redox flow battery (SLFB) builds on the existence of three oxidation states of lead, specifically Pb 0 , Pb 2+ and Pb 4+ [ 26 ]. Chemistry similarity with a lead-acid battery should be noted, except here, the Pb 2+ phase is in soluble form. In the simplest SLFB, Pb 2+ ions are dissolved in methanesulfonic acid (MSA), CH 3 SO 3 H. This ionically conductive solution is pumped into an undivided electrochemical cell with graphitic inert electrodes and the cell is being charged by applied electric current. At the positive electrode, the Pb 2+ ions are oxidized to a solid PbO 2 , which is deposited on the electrode. Simultaneously, at the negative electrode the Pb 2+ ions are reduced to a solid metallic lead [39]. To use the device as a power source, a load is applied on the electrodes, and the opposite reactions proceed. The reactions are as follows: Reaction on the negative plate during charging Pb2+(aq) + 2 e−Pb(s) E0=−0.130 V vs. SHE, Energies 2023,16, 4468 8 of 20 Reaction on the positive plate during charging Pb2+(aq) + 2 H2O(l) PbO2(s) + 4 H+(aq) E0=−1.460 V vs. SHE, Overall reaction 2 Pb2+(aq) + 2 H2O(l) Pb(s) + PbO2(s) + 4 H+(aq) Ecell = 1.590 V MSA was found to be the largest cost component of the studied system, with graphitic bipolar plates next [38]. Lead and lead compounds that find their way into the environment are potential health and environment hazards. Naturally, as the potential for health impairment became more understood, various products have been banned and eventually phased out. Leadacid rechargeable batteries are fundamentally based on the use of lead and lead compounds for their function so far remains an accepted means of storing energy. Their advantage is a long history of use, reliability, universal adoption, safety record and low cost compared to alternatives. That does not mean that lead-acid batteries do not pose with their lead contents environmental and health risks. They do pose a risk, but the risks can be reasonably mitigated [ 26 ]. Health and risk danger from lead-acid batteries during their intended use are really minimal as the operating or stored battery is in its firm enclosure. Thus, the potential and real risks are in the manufacturing of new batteries and their disposal. Considering the value of the material in the retired battery, recycling, rather than disposal, is going to be the method of keeping lead from entering dangerously the environment. Nevertheless, the recycling process has its potential for polluting the environment. For example, a study of soil contamination by lead within and around officially sanctioned lead-acid battery recycling facilities in Africa has been reported [40]. Lead contents separated from the polymer case of the battery, the porous glass matt or other separator and sulfuric acid, are metal lead from the grid and interconnecting flags and the remaining paste from the electrodes, containing lead oxide, led granules and, most significantly, lead sulfate. In the modern environmentally sound approach, lead sulfate has to be first desulfurized in a hydrometallurgical process [41–43]. There are three used methods. (1) Conversion of lead sulfate into lead oxide in alkaline solutions: PbSO4+ 2 OH−PbO + H2O + SO42− (2) Conversion of lead sulfate into lead oxalate in oxalate solutions: PbSO4+ C2O42−PbC2O4+ SO42−; In a similar fashion, citric acid or citrate has been proposed for a similar purpose [ 44 ], where citrate forms strong aqueous complex with lead ion: Pb2+ + C6H5O73−PbC6H5O7 − (3) Conversion of lead sulfates into lead carbonate in carbonate solutions: PbSO4+ CO32−PbCO3+ SO42− The oxalate or citrate route does not require additional reducing agent to process the material to lead, whereas lead carbonate and oxide do. However, since the lead-acid battery industry also requires lead oxides for production of the active mass of the electrodes, thus reclaimed material can be further sold to suppliers of the electrode paste materials. In heat treatment of mixed lead and lead sulfates/oxides recovered from used leadacid batteries, the metallic lead will melt first (328 ◦ C), and lighter solid dross or molten slag will collect on the surface of the molted metal. It can be separated and further processed. Energies 2023,16, 4468 9 of 20 While these procedures can be executed cleanly, to be carried out effectively they have to be conducted on a large scale and with oversight. However, recovering lead can be provided also by informal lead-battery recycling [ 25 ]. This is a common practice around the world, often secretive and very likely without regards to safety of the workers, let alone to the environment. Additionally, the recovery of lead in these informal shops is low; up to one-third of lead may remain unrecovered [ 45 ] and is eventually released into the environment. In the literature describing lead-acid batteries and their manufacture, it is often mentioned, in particular for the USA, that the recycling of the batteries is close to 100%, lowering thus any negative environmental impact. This certainly appears to be reasonable assertion. Purchasing a replacement car battery, for example, in Illinois requires a battery core deposit, a payment above the cost of the battery, if a battery is purchased without returning the old battery at the point of sale right away. Then, within some time limit, one brings the old battery in the store and receives the deposit back. This provides reasonable incentive for the recycling of individual batteries. Stores, repair shops and garages have also incentives to turn in all collected batteries, not least under a threat of fines for violating hazardous material rules. The increase in the percentage of batteries returned to recycling is certainly tied to legislative measures that with the recognition of the potentially dangerous effect of lead on organisms fostered lead end-use patterns. While in the USA the recovery for recycling was about 70% in 1980, it has increased to about 93% in 2000–2003 as many states or communities restricted batteries for disposal at landfills and incinerators [ 46 ]. This has increased to 96% in 2005 and has remained at 99% since 2010 (compare to Table 1[47]). Table 1. Data on lead-acid batteries in municipal solid waste by weight for 1970–2018 (in thousands of US tons) (Adapted from EPS source [47]). Management Pathway 1970 1980 1990 2000 2005 2010 2015 2017 2018 Manufactured 820 1490 1510 2280 2750 3020 2700 2940 2900 Recycled 620 1040 1470 2130 2640 2980 2670 2910 2870 Landfilled 200 450 40 150 110 40 30 30 30 Table 1shows that in recent years, the recycling success has essentially reached 100%. Figure 5shows another promising trend: the amount of lead appearing in municipal waste is slowing down. Recycling rates generally compare the number of batteries estimated to be available for recycling in a given year to the amount of secondary lead produced in that year. However, estimates of recycling rates for lead-acid batteries vary due to many factors, such as identification of which batteries are included in calculations, fluctuation in annual battery sales, time lag in data due to various batteries’ life spans, and imports and exports of batteries and scrap lead. As a result, an accurate battery recycling rate is difficult to establish [ 48 ]. There are also seen dips in recycling success over years that are tied to economy. For example, that trend was observed after successful recycling in the USA during 1970s in the following decade. It was observed that in 1985, due to existing economic conditions of the secondary lead industry coupled with increasingly stringent environmental regulations, a decline has occurred in lead-acid battery recycling rates from typical levels of 80 percent to a level below 60 percent. This translated into approximately 120,000 additional metric tons of lead that exited from the battery recycling chain and entered the environment [49]. Energies 2023,16, 4468 16 of 20 display the recycling history of the two most populous countries in Europe, Germany and France, assuming this would give a good cross-section of the recycling trend. For the dip in 2013 in France for whole batteries we have no explanation, but suspect this may be due to an error in reporting. Interesting is the dip in 2014 in whole batteries in Germany, but not in France. We suspect that the value change is related to the methodology of reporting, as this is the first year lead recovered from batteries has been reported. Some of the data may be commingled. Still, in years 2021–2013, there is clearly a visible decrease in recycling of the whole battery in Germany. Energies 2023, 16, x FOR PEER REVIEW 17 of 21 Figure 8. Recycling of batteries in Germany and France from 2011 to 2020 for the whole lead-acid battery and from 2014 to 2020 for the lead contents (raw data from Eurostat [66]). A noted decrease in the recycling rate of whole batteries happens because polypropylene from ground batteries, where the material of different provenance is mixed, is not suitable for the production of new packaging. In new technological processes, this polypropylene is suitably used as a reducing agent in lead metallurgy. However, since it does not return to production, it is not recorded as recycled. The previous section gives an estimate of lead-acid battery recycling in the US (80% according to EPA [36]). This is a lower figure than in Europe. There is a good battery collection infrastructure in the country. In many states, when buying a new battery, the old one has to be returned or a deposit has to be paid which can be redeemed when a used battery is later returned. However, lower recycling may be due to the fact that the US ships some of the used batteries for recycling to Mexico. It is probably motivated by strict emission regulations from lead factories. Figure 9 shows the increase in the export of used batteries to Mexico in response to tightened emission limits in the US. Figure 9. Export of used batteries from USA to Mexico (raw data from the United States International Trade Commission, tariff code 8548100540. Figure 8. Recycling of batteries in Germany and France from 2011 to 2020 for the whole lead-acid battery and from 2014 to 2020 for the lead contents (raw data from Eurostat [66]). An interesting trend is in a fairly parallel recycling success of the led metal contents in batteries in both countries. It was high, nearly 100%, until 2019, where there is a clear decrease. One explanation put forward explaining this decrease in percent recycling is the export of the spent material for recycling abroad. Thus, the total amount of processed lead waste within the country would decrease. This can certainly be a valid explanation as the export of lead waste and scrap from Germany to India between 2018 and 2019 tripled [ 67 ]. Thus, a competition from the overseas recyclers would have a profound effect on the reported data about domestic recycling. A noted decrease in the recycling rate of whole batteries happens because polypropylene from ground batteries, where the material of different provenance is mixed, is not suitable for the production of new packaging. In new technological processes, this polypropylene is suitably used as a reducing agent in lead metallurgy. However, since it does not return to production, it is not recorded as recycled. The previous section gives an estimate of lead-acid battery recycling in the US (80% according to EPA [ 36 ]). This is a lower figure than in Europe. There is a good battery collection infrastructure in the country. In many states, when buying a new battery, the old one has to be returned or a deposit has to be paid which can be redeemed when a used battery is later returned. However, lower recycling may be due to the fact that the US ships some of the used batteries for recycling to Mexico. It is probably motivated by strict emission regulations from lead factories. Figure 9shows the increase in the export of used batteries to Mexico in response to tightened emission limits in the US. Energies 2023,16, 4468 17 of 20 Energies 2023, 16, x FOR PEER REVIEW 17 of 21 Figure 8. Recycling of batteries in Germany and France from 2011 to 2020 for the whole lead-acid battery and from 2014 to 2020 for the lead contents (raw data from Eurostat [66]). A noted decrease in the recycling rate of whole batteries happens because polypropylene from ground batteries, where the material of different provenance is mixed, is not suitable for the production of new packaging. In new technological processes, this polypropylene is suitably used as a reducing agent in lead metallurgy. However, since it does not return to production, it is not recorded as recycled. The previous section gives an estimate of lead-acid battery recycling in the US (80% according to EPA [36]). This is a lower figure than in Europe. There is a good battery collection infrastructure in the country. In many states, when buying a new battery, the old one has to be returned or a deposit has to be paid which can be redeemed when a used battery is later returned. However, lower recycling may be due to the fact that the US ships some of the used batteries for recycling to Mexico. It is probably motivated by strict emission regulations from lead factories. Figure 9 shows the increase in the export of used batteries to Mexico in response to tightened emission limits in the US. Figure 9. Export of used batteries from USA to Mexico (raw data from the United States International Trade Commission, tariff code 8548100540. Figure 9. Export of used batteries from USA to Mexico (raw data from the United States International Trade Commission, tariff code 8548100540. 7. Conclusions Recycled lead is a commodity of substantial importance, and the need for lead metal over the next decade is expected to be mainly in electrochemical storage batteries, spanning uses in automotive industry, power backups of data centers, large-scale energy storage for power balancing and local transport and load lifting and manipulations. As a technical metal, lead will find its use in electronics (still some remaining solder alloys) and healthcare (radiation shielding). The International Lead Organization [ 68 ] reports that recycled lead fills some 54% of new lead needs worldwide. That number is 74% in Europe, and presently, all lead used in the USA comes for secondary lead production. This high rate is facilitated by legislative pressure to keep lead out of waste, relative ease of collection of scrap metal and economic value in reprocessing the metal. Recycling can also be ecologically friendly since processing of secondary metal can require just 35 to 40% of energy needed to produce primary lead from its ore [ 69 ]. Strong factor is also the declining number of profitably operating lead mines due to lower-yield ore and increased regulations and oversight governing lead extraction, which makes primary lead production a more costly proposition. According to available data, lead battery recycling is high in countries with high GDP. This corresponds to the Kuznets curve [ 28 , 70 ], which predicts that in an economically weak environment, investment will focus on urgent needs. The recycling of lead batteries is in a sense simple because the material is concentrated in a small volume, and the return of used batteries is often financially interesting. Given that lead consumption will continue to rise in the foreseeable future, and that lead reserves are relatively small, recycling will be intensified. Our treatment of the available data confirms that lead production for the purpose of lead-acid battery manufacturing is not slowing down, although our data analysis would be more in line with predictions from the past data by USGS [ 37 ] (Figure 4), rather than the predicted exponential growth shown in Figure 3[ 35 , 36 ]. Reprocessing scrap lead and spent lead-acid batteries remains largely an issue of economy, both on the local level and on the state level. While the operators of the informal shops may be keen on recycling the material and producing rebuilt batteries for profit without much concern for the environment, on the level with more oversight, the large operators will ultimately recycle much of the secondary lead with gradually less and less environmental impact. Author Contributions: Conceptualization, P.B. and P.V.; formal analysis, P.V.; investigation, P.V.; resources, P.B.; data curation, P.B. and P.V.; writing, P.V.; funding acquisition, P.B. All authors have read and agreed to the published version of the manuscript. Energies 2023,16, 4468 18 of 20 Funding: This research was funded by the specific graduate research of the Brno University of Technology No. FEKT-S-23-8286. Data Availability Statement: The data used in this study were obtained from sources by other entities and the sources are referenced in the text. No new data were created in this study. Conflicts of Interest: The authors declare no conflict of interest. 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