Rate-limiting factors for indirect two-step
biodismantling for printed circuit board recycling

Benjamin Monneron-Enaud, Arturo Hernán Bravo-Méndez, Oliver Wiche,

Michael Schlömann

► To cite this version:

Benjamin Monneron-Enaud, Arturo Hernán Bravo-Méndez, Oliver Wiche, Michael Schlömann. Rate-limiting factors for indirect two-step biodismantling for printed circuit board recycling. Resources, Conservation and Recycling, 2025, 218, pp.108253. 10.1016/j.resconrec.2025.108253. hal-05037037

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# Rate-limiting factors for indirect two-step biodismantling for printed circuit board recycling

Benjamin Monneron-Enaud<sup>a,b,*</sup>, Arturo Hernán Bravo-Méndez<sup>a</sup>, Oliver Wiche<sup>a,c,b</sup>, Michael Schlömann<sup>a</sup>

## ARTICLE INFO

### Keywords:

Biodismantling

Bioleaching

Electronic component

Recycling

Printed circuit board

Dismantling

## ABSTRACT

Dismantling of printed circuit boards is a promising pretreatment step for the processing of electronic waste. It allows the separation and subsequent sorting of the different electronic components (ECs) and eases the downstream processes by enriching ECs and the elements contained in them. Biodismantling is a novel process using iron-oxidizing microorganisms to generate an oxidative ferric iron-rich solution to dissolve solder joints, thus releasing ECs. This study evaluates the influence of pH and iron concentration that proved to be rate-limiting factors of the dismantling reaction. Since microbial iron oxidation and dismantling may be performed in separate reactors, it also investigates the effect of sonication on dismantling and thus allows to optimize parameters for industrial applications. At pH 1.2 and 500 mM ferric iron, dismantling times were down to 10 h, but the addition of ultrasound treatment improved dismantling rates even further, reaching complete dismantling within 1.9 h.

## 1. Introduction

Recycling of electronic waste is an important topic as our societies need increasing amounts of metals, including rare and precious metals, which overall puts a high pressure on our environment. Despite the high value of elements in printed circuit boards (PCBs) as a component of electronic waste, their recycling rates remain relatively low. In Europe in 2019, only 42 % of collected electronic waste was formally recycled, with PCB recycling rates reaching up to 61 % (Baldé et al., 2022). In addition, recycling of some of the elements in the PCBs tends to be inefficient by conventional processes of PCB recycling, which are mainly based on smelting. Some of the rare and precious elements have so low concentrations that it is currently not possible to recover them in a cost-efficient way (Wellmer and Hagelüken, 2015). Furthermore, alternative routes by hydrometallurgy still face challenges noticeably regarding metal liberation. In fact, the pretreatment of PCBs which comprises crushing and grinding of the PCBs, is associated with losses of valuable metals in the dust (Chancerel et al., 2009) which reduces the overall efficiency of the process.

Recycling of elements occurring in small amounts or only in certain

electronic components (ECs) in PCBs requires an enrichment step, yielding less complex PCB fractions from which these elements can be recycled. To reduce the complexity of the initial PCB waste, it is possible to dismantle PCBs from their electronic components (ECs) by breaking, melting, or leaching the solder that holds them together. Dismantling is also an opportunity to recover Sn, Pb, Ag, and Cu, which are common constituents of solder material (Bath, 2007). A hydrometallurgical route is of particular interest because of its low capital expenditure and low energy consumption which makes it a potentially more eco-friendly approach (Tuncuk et al., 2012). For this reason, researchers tested different methods of chemical dismantling using various oxidants (Cui and Anderson, 2016) like, for example, H₂O₂ (Zhang et al., 2015), Br₂ (Dorneanu, 2017), Cu²⁺ (Cocchiara et al., 2019), HNO₃ (Yang et al., 2017), or Sn⁴⁺ (Jung et al., 2017), while in other cases the chemistry of the leaching solution was not disclosed (Jiang et al., 2012). All processes using oxidants have the disadvantage of using up the leaching agent, therefore a regeneration of the oxidant would be most beneficial. Thus, Dorneanu (2017) reported an electrochemical reoxidation of the Br⁻ to Br₂ while Cocchiara et al. (2019) used oxygen or electricity to reoxidize the Cu⁺ to Cu²⁺. Fe³⁺ has also been investigated as an oxidant that could

<footnote><sup>a</sup> Technische Universität Bergakademie Freiberg, Freiberg, Sachsen 09599, Germany</footnote>

<footnote><sup>b</sup> Bureau de Recherche Géologique et Minère (BRGM), Orléans 45000, France</footnote>

<footnote><sup>c</sup> Hochschule Zittau/Görlitz, Zittau, Sachsen 02763, Germany</footnote>

<footnote>* Corresponding author at: Bureau de Recherche Géologique et Minère (BRGM), Orléans 45000, France.</footnote>

<footnote>E-mail address: b.monneronenaud@brgm.fr (B. Monneron-Enaud).</footnote>
be regenerated (Yazici and Deveci, 2014). It is especially interesting because it can be regenerated with the help of microorganisms that are usually employed in processes described as biooxidation or bioleaching to obtain metals from minerals (Johnson et al., 2023). Biodismantling is a novel process that uses such iron-oxidizing bacteria to leach solder and dismantle electronic components from PCBs (Monneron-Enaud et al., 2020). The process uses biogenic Fe³⁺ to drive the tin solder corrosion and it uses microorganisms to replenish the Fe³⁺, thus reducing the consumption of chemical for the process. Despite the advantages of this iron regeneration, the process in the first attempts was very slow and needed 20 days to dismantle a PCB. Considering a possible future industrial application, we, therefore, aimed in the present study to understand the parameters that limit the solder dissolution kinetics during biodismantling and to find ways to improve dismantling rates.

## 2. Material and methods

### 2.1. Printed circuit boards

A batch of identical printed circuit boards (Fig. 1) was obtained from an online retailer on eBay. The description mentioned “medical PCBs for Au recovery”. However, their exact functions remain unknown. The PCBs were checked for any obvious mechanical damage before each experiment to keep conditions comparable. Each PCB was 55 mm long, 12 mm wide, and 7 mm deep, weighed 3.7 g, and was equipped with 77 surface-mounted electronic components placed on both sides of the PCB (Fig. 1). Some components could be visually identified as multilayer ceramic capacitors of different sizes or resistors, and a few integrated circuits were present too. One rectangular area and one through-hole were identified to be Au-plated. The total amount of tin was 88.1 mg per PCB (Monneron-Enaud et al., 2020). It is expected to be mainly associated with the solder material. The copper content was determined to be 477 mg per PCB. The detailed elemental composition of the PCB and the EC fractions is given elsewhere (Monneron-Enaud et al., 2020). In this previous work, the absence of Pb indicated that the solder of the PCBs was a lead-free solder. Furthermore, qualitative characterization showed that Sn and Ag were the primary constituents of the solder (data not shown). Therefore, its composition was most likely 96.5 % Sn, 3 % Ag and 0.5 % Cu (Bath, 2007).

### 2.2. Effect of pH value

The effect of pH on the dismantling reaction was investigated in triplicates at four different initial pH values: 1.0, 1.2, 1.5, and 1.8. Experiments were conducted in 250 mL squared flasks containing 200 mL of 15 mM ferric iron (as ferric iron sulfate) solution. The pH was adjusted with sulfuric acid before completing the final volume. One PCB was added per flask just before incubation on a rotary shaker at 30 °C and

<figure><img src="image_4.png" /><figcaption>Fig. 1. Picture of two PCB samples showing the two different faces of the used PCBs prior to any treatment. All electronic components were surface mounted and had exposed solder.</figcaption></figure>

140 rpm for 20 days. Samples were taken every day, by withdrawing 700 µL of liquid which was centrifuged for 5 min at 20,000 x g to separate any precipitate. Of each sample 100 µL supernatant was stored for ICP-MS analysis with the addition of 100 µL 37 % HCl to avoid Sn precipitation. The rest of the supernatant was used for pH and redox potential measurements. Experiments were carried out abiotically but under non-sterile conditions.

### 2.3. Effect of iron concentration and redox potential

To investigate the effect of the iron concentration and the simultaneous effect of the redox potential, experiments were carried out in squared flasks containing iron solutions adjusted to an initial pH of 1.2 with sulfuric acid. Total iron concentrations of 15 mM, 75 mM, and 300 mM were tested at two different redox potentials. For each total iron concentration, two levels of redox potential were set by mixing FeSO₄ and Fe₂(SO₄)₃, corresponding to “low” redox conditions (at start 20 % Fe³⁺) and “high” redox conditions (at start 100 % Fe³⁺), with effective concentrations given in Table 1. A negative control without the addition of iron was set up to verify the effect of solely sulfuric acid. Flasks were prepared in triplicates with a volume of 200 mL and incubated at 30 °C in a rotary shaker at 140 rpm. One PCB was added per flask. Daily sampling of 1 mL was carried out. After centrifugation for 5 min at 20,000 x g, 250 µL supernatant was stored after adding 250 µL 37 % HCl for ICP-MS measurements. The remaining volume was used for the measurements of pH, redox potential, and Fe²⁺ by the Ferrozine® method (Stookey, 1970).

### 2.4. Ultrasound treatment

To test the effect of sonication on the dismantling rate, comparative experiments were conducted. Squared flasks were filled with 200 mL of a 500 mM ferric iron solution with the pH adjusted to 1.2 with sulfuric acid. One triplicate was exposed to ultrasound at room temperature while another was incubated in the rotary shaker at 30 °C. The temperature of the ultrasound condition may have increased throughout the sonication period due to the heat dissipation of the ultrasound energy. A control experiment was also prepared in triplicate by filling the flasks with distilled water adjusted to pH 1.2 with sulfuric acid to investigate the combined effect of sonication and acid in the absence of iron. One PCB was added to each flask at the beginning of the experiment. The ultrasound bath (Transonic 460, Elma Schmidbauer GmbH) had a power of 85 W at 35 kHz and 2 L capacity. The bath was filled with distilled water to the recommended volume when flasks were already placed inside. Because only three flasks could be fitted in the bath, the triplicates for the ultrasound condition and the ultrasound control condition were done sequentially. Each run lasted two hours and thirty minutes at full intensity and continuous ultrasound exposure. Regular sampling was done, for which the flasks were taken out of the bath for evaluation of the dismantling already achieved and for sampling of the solution.

<table><caption>**Table 1**

Effective initial concentrations of ferrous and ferric iron in the experimental setups.</caption><thead><tr><th>Total iron</th><th>Fe³⁺</th><th>Fe²⁺</th></tr></thead><tbody><tr><td>0 mM</td><td>—</td><td>—</td></tr><tr><td>15 mM</td><td>3 mM</td><td>12 mM</td></tr><tr><td>15 mM</td><td>15 mM</td><td>—</td></tr><tr><td>75 mM</td><td>15 mM</td><td>60 mM</td></tr><tr><td>75 mM</td><td>75 mM</td><td>—</td></tr><tr><td>300 mM</td><td>75 mM</td><td>225 mM</td></tr><tr><td>300 mM</td><td>300 mM</td><td>—</td></tr></tbody></table>
## 2.5. Evaluation of the dismantling rate

Dismantling was evaluated by visual inspection and counting of the number of ECs that had fallen off or were left on the PCB. During each sampling, flasks were shaken manually prior to visual inspection. Neither the PCB nor the fallen ECs were removed from the solution during counting. The percentage of dismantling was calculated by dividing the number of fallen ECs by the total number of ECs on the untreated PCB, i.e. 77 ECs per PCB.

## 2.6. Electron-microscopic investigation of sonication on the solder surface

To understand the effect of the sonication on the solder surface, scanning electron microscopic (SEM) analyses were performed on polished sections of solder samples prepared in the following way: To separate the effect of the iron-rich solution on the solder from the effects on other board components like copper, solder beads were prepared using SAC305 solder wire, i.e. 96.5 % Sn, 3.0 % Ag, and 0.5 % Cu alloy, instead of PCBs. The solder wire was first melted with a soldering iron to ensure the removal of the flux chemicals and make the alloy more comparable to the solder on PCBs. The solder was melted at 200 °C and collected as drops in a glass petri dish before cooling at room temperature. The resulting solid solder beads of various sizes were rinsed three times with distilled water to remove residues and then dried on paper.

Four different conditions were prepared to expose the solder beads to Fe³⁺ and sonication. An iron-rich solution was prepared with 500 mM Fe³⁺ and resulted in a solution of pH 1.2. To compare the effect of Fe³⁺ in acid with solely acid, distilled water was adjusted to pH 1.2 with sulfuric acid. Then ten solder beads each of approximately 2 mm diameter were added to flasks containing 200 mL of the respective solution. One flask with the iron-rich solution and one flask with just diluted sulfuric acid were incubated in a rotary shaker at 30 °C for 5 h. For ultrasound treatment, one flask with the iron-rich solution and one flask with diluted sulfuric acid were exposed to continuous ultrasound for 30 min. The ultrasound equipment was used under similar conditions as described in the previous section. After the exposure time, solder beads were separated from the solution, then rinsed with distilled water and dried on paper.

Treated beads were then embedded in an epoxy resin for 24 h before being cut in half to expose the fresh surface. The cross sections were finally polished to obtain a flat surface that allowed the inspection of the

core and surface reaction products. The polished sections were then analyzed in the Geometallurgy Laboratory, TU Bergakademie Freiberg, using the scanning electron microscope FEI Quanta 600 FEG equipped with a Bruker Quantax Dual X-Flash detector 5010. The imaging and the elemental analysis were carried out in the backscattered electron (BSE) mode after a calibration of the greyscale with epoxy resin as the background and gold as the upper limit (Bravo et al., 2024).

# 3. Results

## 3.1. Effect of pH value

The experiments showed that the pH value had a very significant effect on the rates of dismantling (Fig. 2). At pH 1.8, in previous studies (Monneron-Enaud et al., 2020), complete dismantling had been achieved after 20 days. However, in the present study, 20 days was not enough to reach a complete separation of the components at the same pH. The difference between these two attempts can be explained by the NaOH-soaking pretreatment, that had been used in the previous study to liberate the Cu circuits. In the current study, no pretreatment was applied. At pH 1.5, complete dismantling was achieved after 11 days, while 3.9 days were necessary at pH 1.2, and 2.9 days at pH 1.0. The improved dismantling from pH 1.2 to pH 1.0 was only moderate, especially considering that after 2.9 days at pH 1.2, 94 % dismantling had already been achieved. Overall, lowering the pH from pH 1.8 to 1.0 gradually speeded up the rate of ECs falling off the PCB, resulting in a time reduction to less than 1/7th.

During the experimental work, it had repeatedly been observed that minor external forces enhanced the separation of some ECs from the board. Thus, manual agitation before counting was an important step to reduce the deviation between the replicates. However, the large error bars in Fig. 2A represent this dependency, while lower pH improved these variations to narrower ranges. Interestingly, the evolution of the Sn concentrations was not affected by these high standard deviations. An empty PCB is shown as Supplementary Material (Supplementary Fig. 1).

The evolution of the Sn release showed in all cases an initial phase in which the concentration increased, and then peaked, followed by a second phase of a steady decrease, indicating precipitation. Like the time course of dismantling, the rates of Sn dissolution were strongly accelerated by the lowering of the pH values. Also, a higher maximum concentration was achieved in more acidic solutions. Precipitation rates,

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Evolution of the percentage of dismantling (A), and Sn in solution (B) at different pH values (1.0, 1.2, 1.5, 1.8). Each flask contained 200 mL of 15 mM Fe³⁺ solution adjusted to the desired pH with sulfuric acid and one PCB. Flasks were incubated at 30 °C in a rotary shaker. The dismantling percentage was calculated as the number of components detached from the PCB over the total number of components. All experiments were done in triplicate. Data points represent the average with standard deviation as error bars.</figcaption></figure>
interestingly, were distributed in the same way, with pH 1.0 showing the fastest Sn precipitation, followed by pH 1.2, and so on. Therefore, it seemed that the pH influenced the solubility of Sn leading to a faster dismantling at a lower pH.

## 3.2. Effect of iron concentrations

High iron concentrations clearly improved the rates of dismantling (Fig. 3) from 4 days at 15 mM to 1.6 days at 300 mM. The effect of the Fe³⁺/Fe²⁺ ratio varied with increasing concentrations of total iron. At 15 mM total iron concentration and a Fe³⁺/Fe²⁺ ratio of 20/80, dismantling was the slowest, which could be explained by the limited amount of Fe³⁺ available. In fact, each PCB contained 88.1 mg (= 0.74 mmol) of Sn (Monneron-Enaud et al., 2020), so that in 200 mL solution, the maximum possible Sn concentration would have been 3,7 mM. Assuming the Sn in the solution was Sn²⁺, as described in Eq. (1), the total dissolution of Sn in the flask would theoretically require 1.48 mmol ferric iron of which only 0.6 mmol were available. In contrast, with 15 mM of just Fe³⁺ dismantling was considerably faster, and complete dismantling occurred within eight days. At 75 mM total iron, low redox potential (Fe³⁺/Fe²⁺ ratio 20/80) was significantly better than high redox potential (just Fe³⁺). At 300 mM, both redox levels had similar rates of dismantling as the 75 mM 20/80 condition. A further increase of the iron concentration was tested concomitantly with the ultrasound treatment experiment since the triplicate of flasks used as the control condition without ultrasound could be compared to the other iron concentrations. A concentration of 500 mM ferric iron improved the speed further with complete dismantling achieved after 10.7 h.

$$Sn^0 + 2 Fe^{3+} \rightarrow Sn^{2+} + 2 Fe^{2+} \quad (1)$$

The low redox conditions had a great impact on the selective release of Sn in comparison to Cu. A selectivity coefficient could be calculated from molar concentrations (Fig. 3) as described in Eq. (2). Selectivity was calculated only for the initial phase to avoid an effect of the later Sn precipitation on the coefficient.

$$S = \frac{[Sn] - [Cu]}{[Sn] + [Cu]} \quad (2)$$

The selectivity was related to the amount of Fe³⁺ available and the Fe³⁺/Fe²⁺ ratio. Except for the condition where iron was limiting the tin dissolution, i.e. 15 mM 20/80, selectivity descended at different rates. The more iron was available, the more copper was dissolved, thus the less selective the process. But it also seems that the lower the initial redox potential, the slower the copper release thus leading to higher selectivity. This could be interpreted as a difference in kinetics between the Sn and Cu release driven by the lower standard redox potential of Sn²⁺/Sn⁰, -145 mV (vs. SHE), compared to Cu²⁺/Cu⁰, +340 mV (vs. SHE) (Gamsjäger et al., 2012; Zhang and Richardson, 2000).

Sn behavior (Fig. 3B) in general followed a similar trend as in the pH experiment (Fig. 2), showing a peak release followed by precipitation. Surprisingly, the 300 mM just Fe³⁺ condition showed a different trend where Sn solubility seemed impacted by the high redox potential and high iron concentration resulting in an absence of precipitation over the eight days of the experiment. The 500 mM Fe³⁺ condition also did not show any precipitations but the duration of the experiment was shorter. This behavior of soluble tin under similar conditions was repeated several times (data not shown).

The pH evolution showed very similar trends with a decrease in the first day followed by a slower increase over the rest of the experiment. The condition of low redox and high iron concentration (25/75, 300 mM) experienced a higher pH increase than other conditions indicating some proton consumption. The redox potential mirrored the iron ratio set at the beginning of the experiment with the 100/0 conditions starting with the typical 700 mV vs. Ag/AgCl of Fe³⁺ before rapidly decreasing to different levels while the 20/80 and 25/75 ratios started with a redox

potential close to 430 mV. The final levels witnessed again the Fe³⁺-limited behavior of the lowest iron concentration conditions, especially in the case of the lowest Fe³⁺-containing condition (20/80, 15 mM). The redox potential of the control condition (no iron) followed the 300 mM 100/0 conditions which is unlikely to represent the reality. Because the control was systematically measured after the latter condition, a measurement artifact may have occurred due to a memory effect based on the large difference in iron concentration even though good rinsing was performed between samples.

## 3.3. Ultrasound treatment

The ultrasound treatment further improved the rates of the dismantling and decreased the time to achieve complete removal of the ECs from 10 to 2 h, which became the fastest condition achieved (Fig. 4). Two hours might even be an overestimated time since interruption of the ultrasound was necessary for the counting and the sampling procedure, representing a significant amount of time outside the bath and without sonication. Control conditions that contained no iron showed that the sole effect of ultrasound was not sufficient to achieve any dismantling in the same time frame. Also, for the first time, the rate of dismantling correlated with the rates of Sn release, which may indicate that the structure of the solder was better affected in the presence of ultrasound (Fig. 4A, B). In the absence of ultrasound, it was noticed that sometimes some ECs needed a small external force to fall off the board. This may indicate that the solder structure was weakened but not entirely dissolved. Also, it was possible to observe a black color on the surface of the solder incubated in the absence of ultrasound. This indicates the formation of a layer on the surface which may have a passivation effect and would explain the effect of ultrasound. Ultrasound could remove this presumed passivation layer simultaneously as the reaction happened thus optimizing the speed of the reaction.

To investigate the assumption of a passivation layer forming on the solder surface and the associated effect of the ultrasound, analyses were performed on cross-sections of pure solder beads (Fig. 5). In the presence of Fe³⁺ and the absence of ultrasound, a layer of presumably passivating products was formed on the surface of the beads (Fig. 5C, D). Element analyses detected the presence of Ag, Sn-S-O, and Sn-Ag-S-Cu but no Fe. Ag structures could be explained by the nobility of the metal which conferred galvanic protection so that Ag was left over after the dissolution of the Sn. Sn-S-O products may have been passivation products from the reaction of the dissolved Sn with the SO₂⁻ ions from the solution, while Sn-Ag-S-Cu structures could be a mixed product originating from the solder metals and the SO₂⁻ ions. This presumed passivation layer was not homogenous and varied quite a lot in thickness between samples, ranging from 0 to 140 µm. The effect of ultrasound (Fig. 5E,F) clearly reduced the formation of the observed layer to a few micrometers. In this case, no sulfur compounds were detected which confirms the successful avoidance of layer formation. Additionally, the overall surface presented bigger irregularities in the presence of ultrasound which may be associated with cavitation (Fig. 5E). This makes ultrasound a powerful way to provide mechanical stress on the solder structure and improve the dismantling. Control conditions, in the absence of oxidant, seemed unaffected by both the acid and the ultrasound (Fig. 5A, B).

# 4. Discussion

Biodismantling of PCBs has previously been reported as a promising approach for the recycling of chemical elements and electronic components from used PCBs (Monneron-Enaud et al., 2020). However, the process at its concept state, with an incubation time of 20 days until all ECs had fallen off the PCB, was too slow for a practical application. The present study therefore aimed at identifying and overcoming limiting parameters for this process. As summarized in Table 2, the pH and the iron concentration and also the Fe³⁺/Fe²⁺ ratio (i.e. the redox potential) seemed to play a significant role in the solubility and the dissolution and
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (A) Evolution of the percentage of dismantling, (B) Sn concentration in solution, (C) Cu concentration, (D) selectivity index, (E) pH, (F) redox potential, at different starting total iron concentrations and initial Fe³⁺/Fe²⁺ ratios. Each flask contained 200 mL iron solution with the desired initial Fe³⁺/Fe²⁺ ratio and adjusted to pH 1.2 with sulfuric acid. One PCB was added to each flask and incubated at 30 °C in a rotary shaker. Data for the 500 mM condition originate from the control condition of the ultrasound treatment experiment and is also designated as "no ultrasound, 30 °C, 500 mM" in Fig. 4. The dismantling percentage was calculated as the number of components detached from the PCB over the total number of components. All experiments were done in triplicate. Data points represent the average with standard deviation shown as error bars.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Evolution over time of (A) the dismantling percentage and (B) the tin concentration, in the presence and absence of ultrasound. Each flask contained 200 mL of 500 mM Fe³⁺ solution adjusted to pH 1.2 and one PCB. The “no ultrasound” condition corresponds to flasks that were incubated at 30 °C in a rotary shaker while the condition “ultrasound” corresponds to the flasks that were placed in the ultrasound bath at room temperature. The condition “ultrasound, no iron” corresponds to a negative control which contained 200 mL of water adjusted to pH 1.2 with sulfuric acid and treated under ultrasound conditions. All experiments were done in triplicate. Data points represent the average with the standard deviation as error bars.</figcaption></figure>

precipitation rates of the Sn as well as in the rates of dismantling. Furthermore, sonication turned out to be one of the most important impacting parameters to achieve fast dismantling. It allowed to further increase the dismantling rate by a factor 10 (Table 2), overall resulting in dismantling times in the range of hours. With dismantling times in the range of hours, it becomes possible to compare the biodismantling approach with chemical processes, which take between 20 min and several hours under similar temperature conditions (Jiang et al., 2012; Jung et al., 2017; Zhang et al., 2015). Such a reaction speed would be suitable for the design of a two-step biodismantling process making use of iron-oxidizing bacteria to regenerate the oxidizing agent.

In fact, despite the chemical approach of this study, biodismantling aims at using iron-oxidizing bacteria to regenerate ferric iron. Consequently, the choice of pH and iron concentration are important to consider with respect to the microbial capabilities. The most suitable iron-oxidizing microorganisms for this approach are extreme acidophiles, which thrive in a pH range below 3.0 (Baker-Austin and Dopson, 2007). For instance, *Leptospirillum ferriphilum* is a well-known bacterium capable of high-rate iron oxidation at an optimum pH of 1.4 to 1.8 (Coram and Rawlings, 2002). Additionally, *Leptospirillum ferriphilum* has been claimed to be able to oxidize solutions containing up to 500 mM ferric iron (Patel et al., 2012). This was confirmed experimentally in our study (Supplementary Fig. 2). The biogenic oxidation of the Fe²⁺ is thus possible under conditions that are both suited for the bacteria to survive and for the biodismantling to happen. This provides a considerable advantage in comparison to processes that do not use regeneration of the oxidant.

However, the presence of ultrasound is unlikely to produce suitable conditions for the bacterial reaction to happen optimally, as sonication, depending on the power and frequency used, may compromise bacterial integrity and thus survival. Ultrasound is a well-known tool to break bacterial cell walls and the few attempts on one-step sonobioleaching all reported having adverse effects on the microorganism with increasing exposure time, intensities, and frequencies (Vyas and Ting, 2018). Therefore, for a biodismantling process to obtain good dismantling rates and optimal iron biooxidation, it is beneficial to separate the bacterial oxidation reaction from the ultrasound source in two separate reactors. Additionally, the bacterial oxidation may in practice be slower than the iron reduction in the ultrasound bath. This means that to achieve a continuous process a higher hydraulic retention time for iron oxidation and thus a larger volume may be needed.

Under the conditions investigated in the current study, several

interesting effects were noticed as a result of the varying redox potential and iron concentrations. First, under conditions starting with a lower redox potential, the copper dissolved slower than at a high initial redox potential, while Sn dissolution rates were comparable under both conditions (Fig. 3). This effect creates a selective-dissolution situation that enables the favored recovery of Sn. It makes the utilization of the ferric iron more efficient, with most of it contributing to the dismantling, rather than to the dissolution of the Cu. Furthermore, the remaining Cu on the board can be separated from the components and thus can be more easily recovered in downstream processes. Considering that the bacterial iron oxidation should run in a reactor separated from the ultrasound dismantling bath, low redox conditions in the dismantling reactor could be maintained by dosing the amount of ferric iron entering using a redox-controlled system. However, an unselective system can also be imagined in which the redox potential would be maintained at a high level by feeding sufficient Fe³⁺ to allow a maximum dissolution of tin and copper in one step.

Secondly, it was observed repeatedly that under conditions of high Fe³⁺ and sulfate concentrations, e.g. 300 mM 100 % Fe³⁺/0 % Fe²⁺, the redox potential remained above 450 mV (vs. Ag/AgCl) (Fig. 3E), and the tin stayed in the solution and did not precipitate (Fig. 3B). Under high-redox conditions, tin(IV) should dominate due to the more positive redox potential of Fe³⁺/Fe²⁺ as compared to Sn⁴⁺/Sn²⁺ (Gamsjäger et al., 2012; Lemire et al., 2014). Under these conditions, the lack of precipitation of tin can possibly be explained by the formation of an ionic sulfato complex, SnSO₄²⁺, if sufficient sulfate ions are present (Brubaker, 1955). Brubaker (1955) described that with increasing sulfuric acid concentration the Sn⁴⁺ solubility increases (Eq. (3)). However, in the case of this study, it is more likely that the Sn⁴⁺ from the solder gets dissolved and complexed directly by the Fe³⁺ or sulfate ions, respectively (Eq. (4)). This behavior of tin could provide the chance to win the tin by solvent extraction from the leaching fluid. However, with respect to biodismantling, it would mean that the tin would stay in solution, might accumulate there, and at some point would be toxic for the iron-oxidizing bacteria. It has recently been reported that at least Sn²⁺ demonstrates high toxicity towards iron-oxidizing microorganisms gradually inhibiting iron oxidation at concentrations above 3 g/L (Maluleke et al., 2024). Solvent extraction of the tin may reduce the tin toxicity for the bacteria. However, some solvent extractants remaining in the liquid may also affect the iron-oxidation activity of the respective strains (Liu et al., 2022; Mazuelos et al., 1999; Vardanyan and Vyrides, 2019).
<figure><img src="image_8.png" /><figcaption>Fig. 5. Back-scattered electron micrographs of solder bead cross-sections after exposure to (A and B) ultrasound and acidic water pH 1.2 adjusted with sulfuric acid for 30 min, (C and D) to 500 mM ferric iron pH 1.2 for 5 h without ultrasound, and (E and F) after exposure to 500 mM of ferric iron pH 1.2 and ultrasound for 30 min. Red squares in A, C, and E indicate the positions of the detailed micrographs in B, D, and F, respectively. Black-colored background corresponds to the epoxy resin surrounding the beads. The times for taking the micrographs were chosen so that they corresponded roughly to half the time necessary for dismantling, in order to obtain information about the ongoing dismantling process.</figcaption></figure>

$$SnO_2 \ (s) + 2 H_2SO_4 \rightarrow SnSO_4^{2+} + SO_4^{2-} + 2 H_2O \quad (3)$$

$$Sn_0 \ (s) + 4 Fe^{3+} + 2 H_2SO_4 \rightarrow SnSO_4^{2+} + SO_4^{2-} + 4 Fe^{2+} + 4 H^+ \quad (4)$$

Thirdly, pH and redox potential strongly influence the tin precipitation (Figs. 2, 3). Such tin precipitation is helpful in reducing tin toxicity and therefore, it may be favorable to operate a biodismantling process under corresponding conditions. Under low-redox conditions, probably tin(II) prevails and precipitates in the bulk solution away from the solder surface thanks to the difference in kinetics between the dissolution and the precipitation. In the diluted sulfuric acid solution

several different tin(II)-sulfate compounds may theoretically be present, among them SnSO₄, Sn₂OSO₄, Sn₃(OH)₂OSO₄, and Sn₃O₂SO₄ (Ahmed et al., 1998; Edwards et al., 1996; Gamsjäger et al., 2012; Wernfors, 1961). At high redox potential, tin(IV) should prevail, and corresponding precipitates may form. However, the precise compounds are difficult to predict since besides Sn(SO₄)₂, two modifications of Sn(SO₄)₂·2H₂O, Sn(SO₄)₂·4H₂O, and Sn₆O(SO₄)₉ (here average oxidation state of Sn +3.3) have been described as sulfate-containing tin(IV) compounds formed under acidic conditions (Ahmed et al., 1998). Little information is available on the solubility of such tin(IV) compounds (Gamsjäger et al., 2012). In our study, the fastest tin precipitation and the second
<caption>**Table 2**</caption>

Summary of the dismantling and Sn dissolution rates under the different conditions tested. Rates of dismantling were calculated based on the slope of the dismantling evolution between the time zero and time corresponding to 90 % of the total dismantling. Sn dissolution rates were calculated as the slope between the time zero and the pic value of the dissolution. RT corresponds to room temperature starting conditions.

<table><thead><tr><th>Agitation</th><th>Temperature</th><th>[Fe]</th><th>[Fe<sup>3+</sup>]/[Fe<sup>2+</sup>]</th><th>Starting pH</th><th>EC fall rate (EC/days)</th><th>Sn dissolution rate (mM/day)</th></tr></thead><tbody><tr><td>Incubator</td><td>30 °C</td><td>0</td><td>0/0</td><td>1.2</td><td>0.6</td><td>0.01</td></tr><tr><td>Incubator</td><td>30 °C</td><td>15</td><td>100/0</td><td>1.8</td><td>2.8</td><td>0.04</td></tr><tr><td>Incubator</td><td>30 °C</td><td>15</td><td>100/0</td><td>1.5</td><td>9.4</td><td>1.1</td></tr><tr><td>Incubator</td><td>30 °C</td><td>15</td><td>100/0</td><td>1.2</td><td>25.3</td><td>2.3</td></tr><tr><td>Incubator</td><td>30 °C</td><td>15</td><td>100/0</td><td>1.0</td><td>34.3</td><td>2.6</td></tr><tr><td>Incubator</td><td>30 °C</td><td>15</td><td>20/80</td><td>1.2</td><td>6.1</td><td>0.7</td></tr><tr><td>Incubator</td><td>30 °C</td><td>15</td><td>100/0</td><td>1.2</td><td>19.3</td><td>1.9</td></tr><tr><td>Incubator</td><td>30 °C</td><td>75</td><td>20/80</td><td>1.2</td><td>43.9</td><td>2.7</td></tr><tr><td>Incubator</td><td>30 °C</td><td>75</td><td>100/0</td><td>1.2</td><td>24.1</td><td>2.5</td></tr><tr><td>Incubator</td><td>30 °C</td><td>300</td><td>25/75</td><td>1.2</td><td>65.0</td><td>5.3</td></tr><tr><td>Incubator</td><td>30 °C</td><td>300</td><td>100/0</td><td>1.2</td><td>49.9</td><td>5.8</td></tr><tr><td>Incubator</td><td>30 °C</td><td>500</td><td>100/0</td><td>1.2</td><td>203.8</td><td>30.3</td></tr><tr><td>Ultrasound</td><td>RT</td><td>500</td><td>100/0</td><td>1.2</td><td>1968.0</td><td>211.7</td></tr></tbody></table>

fastest dismantling in the absence of ultrasound was observed with 300 mM iron 25 % Fe³⁺/75 % Fe²⁺ (Fig. 3A, B). Thus these low-redox conditions will still allow a relatively quick dismantling and allow the tin to precipitate to avoid any toxic level of tin or solvent extractant. Due to the complexity, no attempt was made to isolate and identify the precipitates or to identify the precise tin species in solution. Independently of the nature of the precipitate, the spontaneous separation of tin from the solution allows to filter this precipitate to recover tin.

A problem remaining after optimization of pH, iron concentration, and redox potential was the apparent formation of passivating layers. Based on the literature, a formation of such layers had to be assumed (Abd El-Rehim et al., 1992; Shitanda et al., 2021). Thus, Abd El-Rehim et al. (1992) by cyclic voltammetry in the presence of sulfate and at various pH values observed two peaks, the one with the more negative potential representing the oxidation of Sn⁰ to Sn(OH)₂ and/or SnO and the one with the more positive potential representing the formation of Sn(OH)₄ and/or SnO₂. Increasing sulfate concentrations resulted in higher peak currents and a shift of the peaks to lower potentials. Interestingly, at low pH (pH 3), the second peak disappeared indicating an inhibition of Sn(IV) formation. In addition to these literature reports, the differences in dismantling rates at the same availability of Fe³⁺ (e.g., 75 mM iron (100 % Fe³⁺/0 % Fe²⁺) versus 300 mM iron (25 % Fe³⁺/75 % Fe²⁺) in Fig. 3A) indicated a formation of passivating layers under the more oxidized (less iron) condition. Such passivating layers can in principle be removed by ultrasound treatment (Bao et al., 2023; Vyas and Ting, 2018). Sonication has been used in a process to separate copper and iron leached from a PCB waste sludge (Huang et al., 2011; Xie et al., 2009). However, the utilization of sonication has rarely been reported in the context of the dismantling. Zhu et al. (2019) reported that sonication of a solution of fluoroboric acid and hydrogen peroxide increased the dismantling rates two- to three-fold. The company Smart Creative Technologies associated with Blue Planet Environmental Solutions Pte Ltd (Smart Creative Technologies, 2024) also mentions the use of ultrasound in their dismantling process using ferric chloride, but the entirety of the process conditions is not disclosed.

Sn is often not a target metal of the bioleaching of electronic wastes since Cu and Au represent most of the value (Baniasadi et al., 2019; Mishra et al., 2021). However, some other less valuable metals like Ni or Al are often mentioned, which could mean that Sn is overlooked for various reasons. In fact, Sn may not be detected in significant concentrations in the leach solution due to its ongoing precipitation during bioleaching. The precipitated Sn mixes with the shredded/ground PCBs and may become part of the residue. In addition, bioleaching is usually carried out at a pH of about 1.8 and at 10 to 100 mM total iron concentration, which leads to a slow recovery rate and again favors precipitation. Hocheng et al. (2014) used a spent-medium approach by using biogenic ferric iron to successfully destroy solder wire in 3 days.

Monneron-Enaud et al. (2020) showed the positive effect of using active iron-oxidizing microorganisms and ferric iron for PCB biodismantling, but the process took 20 days. In light of the current results, it can be assumed that Sn was poorly dissolving as those studies were carried out at a pH of 2.5 or 1.8, respectively.

Biodismantling can overcome the problem of Sn lost in residues by targeting whole boards instead of shredded and ground PCBs. Precipitates of solder residues, and Au particles from the plated area, can easily be recovered by filtration after screening of the fallen ECs and depopulated PCBs. PCBs can then be shredded to be fed to a bioreactor to extract copper by bioleaching (Hubau et al., 2018; 2020) or by one of the other techniques available (Cui and Zhang, 2008). ECs can be sorted to further improve the grade of the different fractions and can then be treated in adapted processes. By such a procedure rare elements contained only in certain ECs can be highly enriched (Monneron-Enaud et al., 2020) and thus become available for recovery rather than being lost in the waste.

## 5. Conclusion

The two-step biodismantling process aims at being an effective and sustainable technique to be applied by the industry. Therefore, the optimization of the dismantling rate is an important step to make the process economic and competitive while keeping its energy efficiency and low environmental impact. Based on the reported results, the pH, the iron concentration, and the application of ultrasound can significantly increase the dismantling rate. Before optimization, the dismantling time was above 20 days, while under optimal Sn dissolution conditions the dismantling time was below 2 h, i.e. 240-fold faster. However, for industrial biodismantling, a process at lower redox potential, with e.g. 25 %Fe³⁺ and 75 % Fe²⁺, though slower, is more realistic since it takes the Fe³⁺ reduction during the dismantling reaction into consideration and it is accompanied by relatively quick tin precipitation which is important for tin removal from the liquid to avoid toxic effects on the iron-oxidizing microorganisms.

The previously reported bioleaching of solder did not give optimal results due to the unfavorably high pH, which is now better understood. Simultaneously, the results of this work explain why the bioleaching of powdered PCBs may initially dissolve the tin from the solder, but still may not be able to recover it because of secondary precipitation into the solid residue. The two-step biodismantling approach, in contrast, should enable the recovery of the Sn and Ag from the precipitate fraction while primarily allowing the separation of the components from the PCBs. A combination of two-step biodismantling and more conventional e-waste bioleaching could be a possible biohydrometallurgical route that would optimize the recovery of Sn and Ag from the precipitate, of precious and critical metals from the EC fraction, and of Cu from the board fraction.
CRediT authorship contribution statement

Benjamin Monneron-Enaud: Writing – review & editing, Writing – original draft, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Arturo Hernán Bravo-Méndez:** Writing – review & editing, Methodology, Investigation, Formal analysis. **Oliver Wiche:** Writing – review & editing, Supervision, Investigation, Formal analysis. **Michael Schlömann:** Writing – review & editing, Validation, Supervision, Resources, Investigation, Funding acquisition, Conceptualization.

Declaration of competing interest

Benjamin Monneron-Enaud, Michael Schlömann, Oliver Wiche have patent #DE102022121684B3 issued to TU Bergakademie Freiberg. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was funded by the generous support of the Dr.-Erich-
Krüger foundation as part of the Biohydrometallurgical Center, and by
Sächsisches Staatsministerium für Wirtschaft, Arbeit und Verkehr
through the innovation fund validation project.

Thanks are due to Prof. Dr. Sabrina Hedrich for her valuable com-
ments on the manuscript, and to Sabine Gilbricht and Dr. Ing. Oleg
Popov for their support with SEM analyses. Many thanks also to Prof. Dr.
Wolfgang Voigt for sharing his insights into tin chemistry.

Supplementary materials

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.resconrec.2025.108253.

Data availability

Data will be made available on request.

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