Urease-Driven Microbially Induced Carbonate Precipitation (MICP) for the Circular Valorization of Reverse Osmosis Brine Waste: A Perspective Review

Abstract

The growing scarcity of freshwater has accelerated the global deployment of desalination technologies, especially reverse osmosis (RO), as an alternative to meet increasing water demands. However, this process generates substantial quantities of brine—a hypersaline waste stream that can severely impact marine ecosystems if improperly managed. This perspective review explores the use of urease-driven Microbially Induced Carbonate Precipitation (MICP) as a biotechnological solution aligned with circular economy principles for the treatment and valorization of RO brines. Through the enzymatic activity of ureolytic microorganisms, MICP promotes the precipitation of calcium carbonate and other mineral phases, enabling the recovery of valuable elements and reducing environmental burdens. Beyond mineral capture, MICP shows promise in the stabilization of toxic metals and potential integration with microbial electrochemical systems for energy applications. This review summarizes current developments, identifies existing challenges, such as microbial performance in saline conditions and reliance on conventional urea sources, and proposes future directions focused on strain optimization, nutrient recycling, and process scalability for sustainable implementation.

1. Introduction

Ensuring an adequate freshwater supply is a pressing global challenge, as freshwater resources are being depleted at an alarming rate [1]. According to the 2024 United Nations World Water Development Report, over 2 billion people already live in countries experiencing high water stress, and nearly 4 billion face severe water scarcity for at least one month each year. These numbers are expected to rise significantly, with an estimated 2.4 billion urban residents projected to face water shortages by 2050, underscoring the urgent need for sustainable alternative freshwater sources [2].

Conventional water resource management has proven insufficient to bridge the gap between water demand and supply. Consequently, the exploitation of unconventional sources such as seawater has become essential, with desalination technologies (DT) offering a stable and scalable solution. DT refers to processes that remove dissolved salts and other minerals from seawater, yielding two products: freshwater, suitable for various applications, and brine, a high-salinity byproduct, also referred to as concentrate or reject, which is typically discharged into marine environments. In the context of transitioning from a linear to a circular industrial economy, the efficient and sustainable management of desalination brine is a global priority and an active area of research.

Most desalination technologies are energy-intensive and include thermal methods such as multistage flash distillation (MSF) and multi-effect distillation (MED), as well as membrane-based technologies like reverse osmosis (RO), which currently dominates the market [3]. According to the International Desalination and Reuse Association (IDRA), approximately 22,000 desalination plants were operational worldwide as of 2023, supplying over 109 million cubic meters of freshwater per day to more than 300 million people. This represents a significant increase from previous years, reflecting the rapid global expansion of desalination capacity in response to escalating water demand [4].

Despite their benefits, DTs raise considerable environmental concerns, primarily due to their high energy consumption. In most desalination plants, energy use accounts for 25% to 45% of the total operating costs. The deployment of renewable energy sources, such as solar, wind, geothermal, and tidal energy, can reduce the carbon footprint associated with fossil fuel combustion. However, brine management remains a significant challenge. Brine is a hypersaline solution that may contain potentially toxic elements (PTEs), including corrosion products, antiscalants, antifouling agents, and halogenated organic compounds. Discharging brine into surface waters can severely damage marine ecosystems, and elevated salinity can induce lethal osmotic shock in marine organisms through irreversible cellular dehydration [5]. In addition to salinity, increases in water temperature (up to 0.7 °C) and the presence of PTEs and chemical residues threaten marine biodiversity [6].

In recent decades, increasing environmental awareness, stricter discharge regulations, and more rigorous salinity thresholds have driven a paradigm shift in the perception of brine from waste to a potential source of freshwater, recoverable minerals, and energy.

This perspective review focuses on the biotechnological potential of Microbially Induced Carbonate Precipitation (MICP) as an innovative solution for valorizing brine produced during reverse osmosis desalination. We present an overview of the MICP mechanism, highlight the key factors affecting its performance, and justify its relevance as part of a circular economy framework. Additionally, we explore alternative urea sources and production techniques, given the critical role of urea in driving the MICP process.

2. Brine Disposal Methods Are Often Environmentally Harmful

Current brine disposal practices include discharge into surface waters, municipal sewer systems, deep-well injection, open evaporation ponds, and land application for irrigation or waste management purposes. These methods contribute to significant environmental challenges worldwide. For instance, sewer discharge of brine can lead to failures in wastewater treatment plants (WWTPs) due to elevated total dissolved solids (TDS) and the inhibition of microbial activity caused by increased salinity. To prevent such failures, brine must be diluted at a ratio of approximately 1:20 [7], which severely limits the feasibility of the method.

Deep-well injection involves storing brine in deep geological formations, typically porous rocks that are sealed by impermeable layers. However, this method carries the risk of groundwater contamination due to fractures caused by seismic activity or human-made perforations [8]. Evaporation ponds, the most commonly used disposal method in arid and semi-arid regions due to abundant solar radiation, facilitate the crystallization of salts for periodic removal. However, this approach requires large land areas and poses environmental risks. If the pond bottom and walls are not properly sealed with geomembranes, hypersaline brine, along with potentially toxic elements (PTEs) and residual chemicals, can leach into groundwater.

Land application of brine involves irrigating salt-tolerant grasses or crops. Although this method may reduce freshwater demand for agriculture, it raises concerns regarding soil degradation and aquifer contamination. Increased soil salinity limits the types of crops that can be cultivated, and infiltrated brine may elevate the groundwater salinity [1]. Despite the widespread use of these disposal methods, concerns remain regarding their long-term impacts on both environmental and human health, highlighting the urgent need for more sustainable alternatives.

Zero Liquid Discharge (ZLD) and Minimal Liquid Discharge (MLD) have emerged as viable alternatives to conventional disposal, enabling the recovery of water, minerals, salts, metals, and energy. These approaches represent advanced strategies in industrial water management, particularly in sectors that generate highly saline effluents, including desalination, mining, and chemical manufacturing. Their adoption is increasingly driven by environmental concerns and stricter regulations regarding brine discharge [9].

ZLD is designed to maximize water recovery—approaching 100%—by converting wastewater into reusable freshwater and transforming the remaining solutes into solid byproducts. A complete ZLD system typically consists of four stages: (1) pretreatment to remove suspended solids, organics, and scaling agents; (2) pre-concentration using membrane processes such as reverse osmosis or nanofiltration; (3) evaporation via thermal technologies like mechanical vapor recompression (MVR) or multi-effect distillation (MED); and (4) crystallization to recover the remaining solutes as solids, thereby eliminating liquid waste [9].

In contrast, MLD systems are less energy-intensive, focusing primarily on pretreatment and membrane-based concentration. They generally recover 90–95% of freshwater while allowing a reduced volume of brine to be discharged or further valorized. Due to its lower energy and capital requirements, MLD offers a more accessible solution for a broader range of applications [10].

However, both technologies face significant challenges. ZLD, in particular, is highly energy-intensive during the evaporation and crystallization stages, which are often powered by fossil fuels, resulting in substantial greenhouse gas emissions and raising concerns about the environmental footprint of these systems. Additionally, the technical complexity and capital investment required for ZLD systems may limit their applicability in remote or economically constrained regions [10,11].

To overcome these barriers, recent research has focused on integrating ZLD and MLD into circular economy frameworks. Coupling these systems with resource recovery strategies—such as salt harvesting, metal extraction, or the use of concentrated brines in biotechnological applications—can enhance both environmental sustainability and economic viability. Moreover, hybrid models incorporating renewable energy sources (e.g., solar-assisted evaporation) or biological treatments are being investigated to further reduce the carbon footprint and improve the efficiency of high-recovery water systems [10,12].

3. Resource Recovery for Brine Valorization

Seawater contains approximately 5×10¹⁶ tons of dissolved salts, a quantity estimated to far exceed the mineral reserves found in all terrestrial mines. It also contains nearly every element on the periodic table in varying concentrations [8]. The most abundant anions and cations in seawater, in decreasing order, are Cl⁻ > SO₄²⁻ > HCO₃⁻ > Br⁻ > BO₃²⁻ > F⁻ and Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ > Sr²⁺, respectively [13,14]. The quantity and composition of desalination brine depend on factors such as feedwater quality, pretreatment processes, desalination technology, and recovery rates. For instance, seawater reverse osmosis plants typically recover 40% to 55% of freshwater, and the resulting brine has approximately twice the salinity of the original seawater [15]. Given these characteristics, desalination brine offers three primary opportunities for resource recovery: freshwater, valuable minerals, and energy.

Desalination technologies used for both freshwater production and brine valorization are broadly classified into two categories: (i) thermal-based and (ii) membrane-based systems [7]. While either category can be used to produce freshwater from raw water, effective brine treatment often requires a combination of both. Zero Liquid Discharge (ZLD), typically achieved through thermal methods, is energy-intensive. Therefore, membrane-based technologies are frequently employed in the early stages to pre-concentrate brine, thereby reducing energy requirements and overall operational costs. However, membrane performance deteriorates when TDS exceeds 70,000 mg/L due to fouling and scaling, which impairs efficiency and increases maintenance needs [16]. Although the extraction of various materials from brine is technically feasible, it remains economically unviable at a commercial scale.

Emerging technologies for recovering high-value strategic elements—such as gold (Au), uranium (U), lithium (Li), bromine (Br), cesium (Cs), magnesium (Mg), and rubidium (Rb)—have been reported [13,17,18]. Abdulsalam et al., (2017) [19] demonstrated that the recovery of sodium (Na), magnesium (Mg), calcium (Ca), and potassium (K) from desalination brine could potentially generate up to US$18 billion annually using solar evaporation ponds. However, these processes are currently limited to laboratory- or pilot-scale applications.

In addition to water and minerals, desalination brine offers the potential for energy recovery. This is primarily based on salinity gradient energy (SGE), which exploits the chemical potential between streams of different salinities, which is a promising renewable energy source for desalination plants [20]. Technologies for harnessing SGE include pressure exchangers and membrane-based systems, such as pressure-retarded osmosis (PRO) and reverse electrodialysis (RED). PRO utilizes salt-rejecting membranes, while RED employs ion-selective membranes to generate electrical energy from salinity differences [21]. Nonetheless, both technologies still face considerable technical and economic barriers before they can be implemented at scale.

Given these limitations, a novel, low-cost, and biologically driven alternative that could partially or completely replace conventional membrane systems—while enabling simultaneous mineral and energy recovery—would significantly reduce the financial and environmental burdens of global desalination operations. This review proposes urease-based Microbially Induced Carbonate Precipitation (MICP) as a circular economy strategy for valorizing brines produced by reverse osmosis desalination. MICP is a naturally occurring process in various ecosystems that contributes to geological phenomena, such as sediment formation and mineral stabilization. Its role in the formation of geological structures and its biotechnological potential are explored in detail in the following sections.

4. Biomineralization and Description of Urease-Based Microbially Induced Carbonate Precipitation (MICP)

Biomineralization is a process by which living organisms facilitate mineral precipitation through biochemical reactions mediated by bacteria, fungi, protists, and plants. It involves various microbial metabolic pathways, such as ureolysis, denitrification, ammonification, and photosynthesis, which influence carbonate formation under favorable environmental conditions [22]. The most extensively studied biomineralization mechanism is urease-driven Microbially Induced Carbonate Precipitation (MICP).

In MICP, ureolytic bacteria hydrolyze urea [CO(NH₂)₂] via the urease enzyme, producing carbonic acid (H₂CO₃) and ammonia (NH₃). NH₃ is then converted to ammonium (NH₄⁺), releasing hydroxide ions (OH⁻), which increases the local pH and creates favorable conditions for mineral precipitation. Concurrently, H₂CO₃ dissociates into carbonate ions (CO₃²⁻), facilitating the precipitation of Ca²⁺ as calcium carbonate (CaCO₃) crystals [23]. The detailed equations are presented below (Equations (1)–(6)).

Urea hydrolysis by the urease enzyme:

$$\mathrm{C}\mathrm{O}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2 \left(\mathrm{a}\mathrm{q}\right)+{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)\to 2\mathrm{N}{\mathrm{H}}_3 \left(\mathrm{g}\right)+\mathrm{C}{\mathrm{O}}_2 \left(\mathrm{g}\right)$$

(1)

Conversion of ammonia into ammonium and pH increase:

$$\mathrm{N}{\mathrm{H}}_3 \left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)\rightleftharpoons {\mathrm{N}{\mathrm{H}}_4}^{+} \left(\mathrm{a}\mathrm{q}\right)+{\mathrm{O}\mathrm{H}}^{-} \left(\mathrm{a}\mathrm{q}\right)$$

(2)

Formation of carbonic acid from carbon dioxide:

$$\mathrm{C}{\mathrm{O}}_2 \left(\mathrm{g}\right)+{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)\rightleftharpoons {\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3 \left(\mathrm{a}\mathrm{q}\right)$$

(3)

Dissociation of carbonic acid and carbonate formation

$${\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3 \left(\mathrm{a}\mathrm{q}\right)\rightleftharpoons {\mathrm{H}\mathrm{C}{\mathrm{O}}_3}^{-} \left(\mathrm{a}\mathrm{q}\right)+{\mathrm{H}}^{+}\left(\mathrm{a}\mathrm{q}\right)$$

(4)

$${\mathrm{H}\mathrm{C}{\mathrm{O}}_3}^{-} \left(\mathrm{a}\mathrm{q}\right)+\mathrm{O}{\mathrm{H}}^{-} \left(\mathrm{a}\mathrm{q}\right)\rightleftharpoons {\mathrm{C}{\mathrm{O}}_3}^{2-} \left(\mathrm{a}\mathrm{q}\right)+{\mathrm{H}}_2\mathrm{O} \left(\mathrm{l}\right)$$

(5)

Calcium carbonate (CaCO₃) precipitation.

$${\mathrm{C}\mathrm{a}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+{\mathrm{C}{\mathrm{O}}_3}^{2-} \left(\mathrm{a}\mathrm{q}\right)\to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3 \left(\mathrm{s}\right) \downarrow$$

(6)

MICP is widely applied due to its low cost, ease of bacterial cultivation and extraction, effective mineralization, and controllable reaction conditions [24]. Over the past two decades, MICP has gained significant attention in the scientific community, resulting in numerous publications exploring its methodologies and applications, including: (i) restoration of calcareous stone materials [25], (ii) bioremediation of soils contaminated with potentially toxic elements [26], and (iii) soil biocementation [27,28]. Despite its promising potential in construction and materials science, the molecular mechanisms of MICP in wastewater and brine environments remain largely underexplored due to the limited number of studies. Nevertheless, the MICP mechanism has been extensively studied in the context of biocementation. At the molecular level, several studies have characterized the structures and regulatory pathways of urease-related genes. Most urease-producing bacteria, including Sporosarcina pasteurii, harbor at least seven genes: ureA, ureB, ureC, ureD, ureE, ureF, and ureG. Among these, ureA, ureB, and ureC encode urease subunits, while ureD, ureE, ureF, and ureG are auxiliary proteins essential for urease activity. Notably, ureE acts as a nickel chaperone, facilitating Ni²⁺ incorporation into the urease active site [29].

Although preliminary findings have been reported, the efficient implementation of MICP depends on the interplay between biological and chemical factors. The key parameters include: (i) pH, (ii) temperature, (iii) dissolved oxygen concentration, (iv) concentrations of calcium, urea, and dissolved inorganic carbon (DIC), (v) medium composition, and (vi) bacterial properties, such as urease activity, cell density, and microbial bioavailability [23,29,30].

NH₃ release leads to a local pH increase due to OH⁻ generation (Equation (2)); however, the dissociation of carbonic acid into bicarbonate and protons (Equations (4) and (5)) introduces H⁺ ions that may buffer this effect on pH. Therefore, the net pH shift depends on the balance between these opposing reactions. Most ureolytic bacteria grow optimally between pH 8 and 10, with growth inhibition typically occurring above pH 10 [31]. Within this moderately alkaline range, urease activity remains sufficient to sustain urea hydrolysis and drive pH elevation, favoring the conversion of HCO₃⁻ to CO₃²⁻ and promoting CaCO₃ precipitation. Carbonate ions may also co-precipitate with other divalent metals, such as Mg²⁺, Sr²⁺, and Pb²⁺, forming poorly soluble carbonates. These reactions are thermodynamically favored due to the low solubility product constants (Ksp) of the resulting compounds, contributing to metal immobilization. For instance, in Cupriavidus sp. W12, Ca²⁺ removal was most efficient at pH 8.0, illustrating the importance of pH control rather than suggesting new carbonate chemistry insights [32,33]. These findings support the application of MICP for the precipitation and immobilization of elements such as lead (Pb), cadmium (Cd), and arsenic (As), reducing their bioavailability and toxicity and minimizing environmental dispersion [34,35]. A similar mechanism may occur with ions in reverse osmosis brine, enabling the precipitation of minerals depending on the specific ionic composition (Figure 1) [36].

Bacterial surface properties and cell concentration significantly affect CaCO₃ precipitation. While S. pasteurii is the most studied MICP bacterium, other genera, such as Bacillus, Halomonas, Shewanella, Citrobacter, Exiguobacterium, Lysinibacillus, and Kocuria, have demonstrated similar potential, as have fungi like Aspergillus niger [31,37].

The bacterial cell wall plays a crucial role due to its negatively charged functional groups that attract cations, such as Ca²⁺. In addition, extracellular polymeric substances (EPS) contribute significantly to MICP (Figure 1). EPS are anionic biopolymers enriched with functional groups, such as carboxyl, phosphate, and hydroxyl, which bind divalent cations, enhance local supersaturation, and promote CaCO₃ nucleation [38]. Over the past two decades, microbial extracellular polymeric substances (EPS) have gained significant attention due to their role in mediating calcium carbonate precipitation. EPS helps retain ions and metabolites near cell surfaces, increasing the likelihood of precipitation [39] and influencing crystal morphology, size, and polymorphism through interactions with both organic and inorganic additives [40]. Moreover, EPS enhances the microbial biofilm structure, creating a stable microenvironment that supports pH buffering and cell viability during ureolytic activity [41]. EPS from different microbial sources show variable adsorption capacities, as functional groups such as carboxyl, hydroxyl, and amino groups serve as efficient binding sites for heavy metals, facilitating bioprecipitation [42].

Okwadha et al., (2010) [43] emphasized that the optimization of MICP depends not only on the appropriate combination of microbial strain, urea, and calcium source but also on bacterial density. Their results indicate that, under non-limiting substrate conditions, cell concentration has a greater impact on urease activity than the initial urea concentration [43].

Temperature is another key factor that affects both bacterial growth and urease activity. Optimal performance occurs near 30–37 °C for mesophilic bacteria, such as S. pasteurii. Deviations from this range reduce cell viability and enzyme efficiency, thereby limiting CaCO₃ production [34]. Temperature also influences the stability of bacterial cell walls and the activity of the surface functional groups [44]. Wang et al., (2022) showed that while urease activity remained stable at 4 °C, CaCO₃ formation was reduced due to decreased bacterial growth and slower kinetics. Conversely, at 50 °C, urease activity declined rapidly [45].

Urea and calcium ion concentrations are also critical factors. Negatively charged microbial surfaces attract Ca²⁺, providing nucleation sites for precipitation [46]. Lv et al., (2023) evaluated the effects of different calcium sources—calcium acetate, nitrate, and chloride—finding that calcium acetate led to higher carbonate content and vaterite predominance [47]. Thus, further strategies are needed to enhance MICP under various calcium concentrations and sources. Reverse osmosis brine offers a promising resource. According to Ihsanullah et al. (2022) [13], these brines contain calcium levels ranging from 669 to 960 ppm, which are sufficient for biocementation and mineral recovery.

Regarding urea sources, Danjo and Kawasaki (2016) reported that coastal urea from the biodegradation of fish and animal waste could serve as a carbon source [48]. Comadran-Casas et al. (2022) used urine-derived urea in MICP experiments and concluded that (i) urea in cow urine remains stable and suitable for MICP, (ii) the soil response to cow urine within pH 7–9 is comparable to chemical solutions, and (iii) increased pH enhances ureolytic activation and carbonate content [49].

5. Perspectives on MICP as a Circular Economy Strategy for Brine Valorization

The circular economy (CE) concept, which focuses on transforming waste into resources, is defined as the reuse, repair, refurbishment, and recycling of existing materials and products—where what was previously considered waste becomes a resource [50]. This perspective review proposes the valorization of reverse osmosis (RO) brine through the application of urease-based MICP, as illustrated in Figure 2. During the MICP process (Step 1), NH₄⁺ and CO₃²⁻ are generated through urea hydrolysis catalyzed by ureolytic bacteria via urease secretion. The resulting increase in pH promotes biomineral precipitation, which can be recovered as valuable components under the concept of “remine”, which refers to the recovery of raw materials from discarded waste.

These newly obtained resources (Step 2) can be reused across various applications or serve as substitutes for conventionally extracted minerals. Furthermore, the ammonium-rich residual brine left after the MICP process can be repurposed for mineral processing or energy recovery by employing microbial electrochemical cell (MEC) systems (Step 3).

Incorporating circular economy principles into mining waste management offers a significant opportunity to reduce environmental liabilities while increasing resource values. In this context, repurposing (using discarded materials for new functions) and recycling (reprocessing to reduce the demand for new resources) are key strategies for value retention. The mining sector must create new economic value, minimize social and environmental impacts, and reduce liabilities related to waste. Recovering valuable components from desalination brine not only mitigates its environmental burden but may also help offset desalination costs by generating additional revenues. The following scientific articles provide evidence supporting this perspective.

The first documented attempt was by Hammes et al., (2003) [51], who demonstrated that urease-based MICP could remove Ca²⁺ from industrial wastewater (489 ± 39 mg/L) with a removal efficiency exceeding 90%. Hu et al., (2021) [52] subsequently evaluated the process for treating hypersaline hydraulic fracturing wastewater (TDS = 129.3 g/L), but found that high salinity inhibited microbial activity, limiting Ca²⁺ removal to <13%. However, dilution with municipal wastewater (1:1 ratio) increased Ca²⁺ removal to ~96%.

If dilution is required to improve microbial performance, it can be strategically integrated into the treatment process using minimal amounts of freshwater or alternative sources such as treated wastewater or greywater. While traditional ureolytic strains may require dilution, the proposed strategy focuses on identifying or engineering halophilic ureolytic bacteria capable of functioning directly in hypersaline conditions, thereby reducing the need for additional water inputs. A key challenge is the adaptation of ureolytic bacteria to high salinity. Sporosarcina pasteurii, a model organism widely used in MICP, is not inherently halophilic; however, it exhibits a high level of tolerance to salinity. Huang et al., 2022 [53] showed that Sporosarcina pasteurii can stay active in seawater and still precipitate calcium carbonate, even though high salinity slows its growth, changes cell shape, and dampens the overall biomineralization rate. When this bacterium was added to mortar specimens, the CaCO₃ produced under saline conditions was sufficient to seal more than 80% of the initial cracks within 15 days, sharply reducing water permeability. Hence, even with partially inhibited biomineralization, the process proceeds fast enough to achieve the self-healing performance targeted for bio-based mortars in marine environments [53]. However, its physiological performance may decline under extreme saline conditions, such as those found in RO brines. Therefore, identifying halophilic urease-producing strains is essential. Xiao et al., (2022) reported that seawater initially inhibits MICP activity in S. pasteurii, though this effect can be mitigated through adaptive culturing [54].

MICP also effectively removes potentially toxic elements (PTEs), with reported efficiencies of ~100% for As, Cd, Mn, and Ni, 92.2% for Ba, and 94.2% for Sr. Hu et al. (2021) [52], NH₄⁺ (2069 mg/L) was successfully removed via a diffused-aeration stripping and scrubbing system, achieving 98% removal and a projected profit of $0.80/m³ of the treated wastewater. Bai et al., (2021) [55] demonstrated Pb²⁺ removal from saline solutions using Exiguobacterium sp. JBHLT-3, showing that calcite had a higher Pb²⁺ uptake than vaterite. While higher salinity increased the vaterite fraction, it reduced the Pb²⁺ removal efficiency, which was still 89% at 12% salinity. Other studies have used Rhodococcus erythropolis TN24F to recover struvite and Bacillus subtilis LN8B to induce hydromagnesite and aragonite formation from seawater ions [56,57].

Partila et al., (2025) [58] showed that irradiated Micrococcus luteus produced a robust EPS matrix that immobilized Ca²⁺ and scaffolded calcite growth. Spectroscopic, microscopic, and thermogravimetric analyses confirmed the presence of amine, hydroxyl, and carboxyl groups in EPS, which enhanced mineral aggregation and bioprecipitate cohesion [58].

Although a few recent studies (e.g., Partila et al., 2025 [58]) have explored the use of MICP for mineral recovery from RO brine, this application remains underrepresented. Further research is required to unlock its full potential in circular economy frameworks.

An important consideration is the NH₄⁺-rich effluent. Each mole of hydrolyzed urea releases two moles of NH₄⁺. At high pH and temperature, NH₄⁺ primarily exists as NH₃. By adjusting the pH and temperature, NH₃ release can be minimized [59]. Other approaches include coupling MICP with ammonium recovery via the Bazarov reaction [60], anammox process [61], or the use of adsorbent materials [62]. Nevertheless, NH₄⁺ is the main source of fertilizer production; therefore, this compound can be removed from water and simultaneously recovered, thus reducing the operating cost of biological-based technology. Mohsenzadeh et al., (2022) [63] investigated a two-stage treatment process for managing ammonium byproducts generated after MICP. This approach involved soil rinsing to remove ammonium, followed by its recovery as struvite. The study demonstrated that 86.8% of ammonium ions could be recovered as high-purity struvite (~94%). Optimal recovery was achieved using Na₂HPO₄ as the phosphate source, with a molar ratio of Mg²⁺:NH₄⁺:PO₄³⁻ of 1.2:1:1 and pH of 8.5. An interesting study by Williamson et al. (2021) [64] evaluated the leaching potential of biogenic ammonium produced by a ureolytic strain of Lysinibacillus sphaericus applied to various waste materials. The process achieved moderate to high metal recovery yields (30–70%) and exhibited very high selectivity for copper and zinc over iron (>97%) using 1 mol L⁻¹ total ammonium. Ammoniacal leaching solutions have been widely recognized as effective lixiviants for the oxidative dissolution of diverse mineral phases [49,50]. Therefore, the presence of NH₃/NH₄⁺ in the effluent could be not only manageable but also potentially advantageous, offering added value in the implementation of the MICP process for desalination brine treatment.

Although ureolysis is not a redox process, the bacteria involved in MICP may engage in additional metabolic pathways that generate electrons when metabolizing organic or nitrogenous compounds. These electrons can be harvested using microbial electrochemical technologies (METs), such as microbial fuel cells (MFCs) and microbial electrolysis cells (MECs), enabling further valorization of residual brine [65].

METs are classified into several variants: (i) microbial fuel cells (MFCs), (ii) microbial electrolysis cells (MECs), where ‘E’ refers to electrolysis for hydrogen production, and (iii) microbial desalination cells (MDCs). In general, electroactive bacteria in the anodic chamber oxidize organic matter, producing electrons and protons. Protons are transferred through a proton exchange membrane to the cathodic chamber, while electrons flow through an external circuit, ultimately generating energy in the form of electricity [66].

The main reaction mechanisms in MFCs can be analyzed using models based on electrochemical principles. In these models, electrical performance—such as output voltage and current density—is commonly described using fundamental electrochemical theory. The most frequently used formulations are based on Nernst–Monod kinetics or Nernstian behavior expressed through Michaelis–Menten-type functions [67].

The direct use of seawater or brine in electrolysis is a promising route for hydrogen production, as these resources are abundant and do not require freshwater [68]. In this context, ammonia has gained attention as a potential hydrogen carrier due to its high energy density and ease of storage and handling. Notably, ammonia electrolysis requires a theoretical external voltage of only 0.06 V, which is significantly lower than the 1.23 V required for water electrolysis [69]. Using a biogenic NH₃/NH₄⁺ electrolyte, the production of H₂ is enhanced through the electrolysis of saline electrolytes. This alternative would significantly support the decarbonization process by offering a disruptive route for hydrogen transport. This choice opens a range of opportunities for the use of saline electrolytes, such as seawater, brine derived from reverse osmosis, geothermal waters, and altiplano brines, particularly those found in the Atacama Desert, which hosts extensive natural reservoirs of hypersaline waters rich in salts and ammonium.

Urea is the primary reactant for the MICP process. However, the use of urea as a nitrogen source is not inherently circular unless recycling systems or alternative nitrogen sources are implemented. Therefore, exploring alternative sources of urea, including its extraction from animal or human urine, is highly advantageous. For instance, Chen et al., 2019 [70] applied pig urine (PU) to sand columns to drive MICP. Their study demonstrated that PU can effectively substitute commercial urea, facilitating CaCO₃ precipitation in the presence of Sporosarcina pasteurii. The quartz-sand experiments revealed that shorter injection intervals (4 h) increased CaCO₃ formation by 43% compared to the controls, improving the mechanical properties. XRD and SEM confirmed mineral formation, highlighting PU’s potential to reduce MICP costs, mitigate environmental waste, and promote sustainable biotechnological applications [70]. Another emerging alternative is urea production via electrochemical methods.

Industrial urea synthesis typically involves a two-step process (Figure 3). First, ammonia is produced through the Haber–Bosch process (N₂ + H₂ → NH₃) under extreme conditions (350–550 °C and 150–350 bar). Then, ammonia reacts with CO₂ [NH₃ + CO₂ → CO(NH₂)₂] under moderate conditions (170–200 °C, 200–250 bar). This process also enables the synthesis of valuable organonitrogen compounds, such as methylamine, ethylamine, and acetamide [71]. However, it is highly energy-intensive and a major contributor to greenhouse gas emissions.

However, in this method, achieving high-purity NH₃ extraction from the electrolyte remains a significant challenge. Additionally, the effective fixation of CO₂ onto the catalyst surface, followed by its interaction with NH₃ to form urea, represents a major obstacle. Overall, the conventional two-step process for large-scale urea production is highly energy-intensive and contributes substantially to greenhouse gas emissions. In contrast, an electrochemical route that directly co-reduces N₂ and CO₂ in aqueous media under ambient conditions—[N₂ + CO₂ + 6H⁺ + 6e⁻ → CO(NH₂)₂ + H₂O] (see Figure 3)—using a highly efficient electrocatalyst, presents a promising pathway for future sustainable urea synthesis [72].

The increasing demand for nitrogen-based fertilizers has driven the development of more sustainable urea production methods. Electrocatalysis has emerged as a viable alternative, allowing urea synthesis under milder conditions while harnessing renewable energy sources, such as solar power. The electrochemical conversion of N₂ and CO₂ into urea under ambient conditions offers an innovative and environmentally friendly strategy for producing green urea [73].

Building on these advances, the electrocatalytic transformation of CO₂ also enables the synthesis of high-energy-density liquid fuels, simultaneously addressing energy security and carbon neutrality. During the electrochemical reduction of CO₂, carbon monoxide electrolysis has emerged as a promising route for generating multicarbon compounds via carbon–carbon coupling. C–N bond formation can occur through the nucleophilic attack of ammonia on the *C=C=O intermediate, yielding acetamide with an efficiency of nearly 40%. This demonstrates a feasible approach for C–N bond formation and amine synthesis through the co-electrolysis of carbon- and nitrogen-containing feedstocks.

A direct one-step conversion of CO₂ and N₂—bypassing conventional ammonia production—could revolutionize urea synthesis by providing a more efficient and sustainable alternative [74]. Density functional theory (DFT) calculations have been used to evaluate the catalytic activity of three experimentally available two-dimensional metal borides (MBenes)—(i) Mo₂B₂, (ii) Ti₂B₂, and (iii) Cr₂B₂—for the simultaneous electrocatalytic coupling of N₂ and CO₂ to produce urea under ambient conditions. Unlike MXenes, where surface metal atoms typically bind to three carbon or nitrogen atoms, M₂B₂-type MBenes exhibit a higher coordination number for surface metal atoms, providing enhanced stability and distinct electronic properties than MXenes.

The optimized lattice parameters and characteristic bond lengths of Mo₂B₂, Ti₂B₂, and Cr₂B₂ are crucial for their catalytic performance. The mechanism for electrochemical urea synthesis via N₂ and CO₂ coupling can be divided into four key stages: (1) adsorption of N₂ and CO₂, (2) electrochemical reduction of *CO₂ to *CO, (3) coupling of *N₂ and *CO to form the key *NCON intermediate, and (4) hydrogenation of *NCON to yield urea. For successful urea electrosynthesis, both N₂ and CO₂ must be effectively adsorbed on the catalyst surface. Moreover, the reduction of adsorbed N₂ to NH₃ must be suppressed to favor urea formation. Once N₂ is adsorbed, CO₂ must be efficiently activated to proceed along the desired pathway (Figure 4) [72,73].

The electrocatalytic synthesis of urea typically results in a complex mixture of byroducts, including ammonia (NH₃), hydrogen (H₂), methylamine (CH₃NH₂), ethylamine (C₂H₅NH₂), acetamide (CH₃CONH₂), carbon monoxide (CO), methane (CH₄), ethylene (C₂H₄), formic acid (HCOOH), methanol (CH₃OH), and ethanol (C₂H₅OH). Among these, gaseous byproducts such as H₂, CO, CH₄, and C₂H₄ can be efficiently analyzed using gas chromatography (GC), allowing the precise identification and quantification of reaction intermediates and side products [71,75].

The key reaction pathways for urea photo/electrosynthesis are summarized below [76]:

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{N}}_2+{6\mathrm{H}}^{+}+{6\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{O}{\mathrm{N}\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O};E^0=0.211\mathrm{V}$$

(7)

$${\mathrm{C}\mathrm{O}}_2+\mathrm{N}\mathrm{O}+{10\mathrm{H}}^{+}+{10\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{O}{\mathrm{N}\mathrm{H}}_2+{3\mathrm{H}}_2\mathrm{O};E^0=0.772\mathrm{V}$$

(8)

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{N}\mathrm{O}}_2^{-}+{16\mathrm{H}}^{+}+{14\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{O}{\mathrm{N}\mathrm{H}}_2+{5\mathrm{H}}_2\mathrm{O};E^0=0.833\mathrm{V}$$

(9)

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{N}\mathrm{O}}_3^{-}+{18\mathrm{H}}^{+}+{16\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_2\mathrm{C}\mathrm{O}{\mathrm{N}\mathrm{H}}_2+{7\mathrm{H}}_2\mathrm{O};E^0=0.811\mathrm{V}$$

(10)

The main competition reactions are as follows

$${\mathrm{C}\mathrm{O}}_2+{2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O};E^0=-0.106\mathrm{V}$$

(11)

$${2\mathrm{H}}^{+}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_2;E^0=0\mathrm{V}$$

(12)

$$N_2+{8H}^{+}+{6e}^{-}\to {2NH}_4^{+};E^0=0.275\mathrm{V}$$

(13)

$$\mathrm{N}\mathrm{O}+{6\mathrm{H}}^{+}+{5\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_4^{+}+{\mathrm{H}}_2\mathrm{O};E^0=0.836\mathrm{V}$$

(14)

$${\mathrm{N}\mathrm{O}}_2^{-}+{8\mathrm{H}}^{+}+{6\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_4^{+}+{2\mathrm{H}}_2\mathrm{O};E^0=0.897\mathrm{V}$$

(15)

$${\mathrm{N}\mathrm{O}}_3^{-}+{9\mathrm{H}}^{+}+{8\mathrm{e}}^{-}\to {\mathrm{N}\mathrm{H}}_4^{+}+{3\mathrm{H}}_2\mathrm{O};E^0=0.875\mathrm{V}$$

From a thermodynamic standpoint, the coupling of CO₂ with N₂, NO, NO₂⁻, or NO₃⁻ occurs at positive standard reduction potentials (pH = 0), indicating favorable conditions for urea formation. Notably, the co-reduction of CO₂ and N₂ follows a relatively low positive potential, suggesting high feasibility. From a kinetic perspective, synthesis routes involving NO, NO₂⁻, or NO₃⁻ require two sequential C–N coupling steps, whereas the direct formation of the *NCON intermediate from CO₂ and N₂ involves a single step, making it a simpler pathway. Moreover, multiple studies have shown that the photo/electrocatalytic coupling of CO₂ with NOx species generally requires lower reduction potentials than molecular N₂, supporting the viability of these routes under mild conditions [76,77].

6. Future Outlook and Conclusions

MICP is a promising biotechnological strategy for addressing the environmental challenges posed by RO brine disposal. By leveraging ureolytic bacteria, MICP enables the precipitation of valuable minerals while reducing the ecological footprint of the desalination process. Its integration into circular economy frameworks could help transform brine from waste into a source of raw materials, clean energy, and freshwater. To enable practical implementation, several critical areas must be addressed. First, the isolation or engineering of halophilic ureolytic strains is essential for ensuring high-performance biomineralization under hypersaline conditions. These microorganisms must combine robust urease activity with tolerance to high ionic strengths and osmotic stress. Second, the environmental sustainability of MICP depends on sourcing urea responsibly. Alternatives, such as urine recycling and electrochemical synthesis, offer circular options to reduce reliance on industrial urea production. In parallel, the management of ammonium-rich effluents is essential. This can be achieved through nutrient recovery, integration with microbial electrochemical technologies (METs), or use in secondary processes, such as hydrogen production. Ultimately, MICP has the potential to become a cornerstone of sustainable brine management. Bridging microbial biotechnology, electrochemistry, and resource recovery offers a path toward environmentally responsible desalination. Continued research on microbial performance, urea alternatives, and integrated process design will be key to scaling this approach for real-world applications.