Surfactin As a Green Agent Controlling the Growth of Porous Calcite Microstructures

International Journal of Molecular Sciences

Article Surfactin as a Green Agent Controlling the Growth of Porous Calcite Microstructures

Anna Bastrzyk 1,*, Marta Fiedot-Toboła 2,* , Halina Maniak 3 , Izabela Polowczyk 1 and Gra˙zynaPłaza 4 1 Department of Process Engineering and Technology of Polymer and Carbon Materials, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrze˙zeWyspiańskiego27, 50-370 Wrocław, Poland; [email protected] 2 Łukasiewicz Research Network-PORT Polish Center for Technology Development, Stabłowicka 147, 54-066 Wrocław, Poland 3 Department of Micro, Nano and Bioprocess Engineering, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrze˙zeWyspiańskiego27, 50-370 Wrocław, Poland; [email protected] 4 Department of Environmental Microbiology, Institute for Ecology of Industrial Areas, Kossutha 6, 40-844 Katowice, Poland; [email protected] * Correspondence: [email protected] (A.B.); marta.fi[email protected] (M.F.-T.); Tel.: +48-71-320-32-39 (A.B.); +48-71-734-71-54 (M.F.-T.) Received: 8 July 2020; Accepted: 31 July 2020; Published: 1 August 2020

Abstract: This study presents a new, simple way to obtain mesoporous calcite structures via a green method using an eco-friendly surface-active compound, surfactin, as a controlling agent. The effects of synthesis time and surfactin concentration were investigated. The obtained structures were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) coupled with gas mass spectrometry (QMS) analysis. The experimental data showed that surfactin molecules significantly changed the morphology of the calcite crystals, roughening and deforming the surface and creating a greater specific surface area, even at low biosurfactant concentrations (10 ppm). The size of the crystals was reduced, and the zeta potential value of calcium carbonate was more negative when more biosurfactant was added. The XRD data revealed that the biomolecules were incorporated into the crystals and slowed the transformation of vaterite into calcite. It has been shown that as long as vaterite is present in the medium, the calcite surface will be less deformed. The strong influence of surfactin molecules on the crystal growth of calcium carbonate was due to the interaction of surfactin molecules with free calcium ions in the solution as well as the biomolecules adsorption at the formed crystal surface. The role of micelles in crystal growth was examined, and the mechanism of mesoporous calcium carbonate formation was presented.

Keywords: biomineralization; calcite; vaterite; biosurfactant; Turbiscan; TGA; XRD

1. Introduction

Calcium carbonate (CaCO3) is a common biomaterial produced by living organisms for building shells and exoskeletons such as seashells, avian eggshells, and bones to support and protect their bodies [1]. This biomaterial possesses unique properties (high surface area, high porosity, high mechanical strength, and non-toxicity) that can offer wide potential applications in industry as filler materials for paints, pigments, coatings, paper and plastics, carriers of active compounds and nanoparticles for drug delivery, and matrices for polymeric capsules [2–7]. CaCO3 occurs in different crystalline polymorphs: anhydrous phases of aragonite, vaterite and calcite, and hydrated phases of calcium carbonate monohydrate, calcium carbonate hexahydrate and amorphous calcium carbonate [8]. Each of these crystalline forms is characterized by various morphologies and physicochemical properties. For instance, spherical hexagonal

Int. J. Mol. Sci. 2020, 21, 5526; doi:10.3390/ijms21155526 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 5526 2 of 15 vaterite exhibits higher porosity and can decompose rapidly under relatively mild conditions compared with rhombohedral calcite [2]. Aragonite, on the other hand, forms thin needles and has a strong tendency to form spherulitic clusters with high porosity and density [9]. In biology, biomineralization can occur at specific sites on the cell wall, in spaces between closely packed cells, inside enclosed compartments within a cell or in spaces outside a cell. The specific mineralization sites can include phospholipid vesicles, protein vesicles, cellular assemblies, and molecular frameworks [10]. The formation of crystalline polymorphs with desired properties is controlled by different and occasionally synergic processes, such as the regulation of respective ion concentrations, the formation of initial amorphous and other metastable precursor phases, and the presence of specific biomolecular additives [11]. Extensive research has shown that living organisms require biomolecular additives such as proteins and polysaccharides with carboxyl, phosphate and sulfate groups to control the hierarchical structure and the mechanical properties of calcium carbonate [10,12]. These biomolecules can act as templates and control the polymorphs, orientation and morphology of calcium carbonate. Thus, research has been focused for many years on learning and mimicking the biosynthesis of calcium carbonate. The ability to control the particle shape, size and morphology is fundamental from the viewpoint of technical applications [13]. Therefore, finding a way to obtain calcium carbonate with specific properties remains a challenge, especially with the use of “green additives”. These “green compounds” include biosurfactants, the surface-active molecules produced by living cells. Compared to synthetic surfactants, biosurfactants are non-toxic, easily biodegradable and can be synthesized from renewable waste with an appropriate ratio of carbohydrates and lipids to support optimal bacterial growth [14]. Surfactin (C53H93N7O13) is an anionic lipopeptide biosurfactant with two carboxyl groups that has been widely used in the “green synthesis” of silver and gold nanoparticles [15]. Our previous research has shown that surfactin can also be used effectively as a calcium carbonate growth controlling agent. CaCO3 structures obtained in the presence of surfactin were characterized by large irregularities (terraces and depressions) on the surface of the crystals during a short ageing period [16]. To better understand the mechanism of crystal growth, this study investigated how surfactin concentration can affect the morphology, polymorphs and properties of calcium carbonate precipitated in an aqueous solution after prolonged ageing. The kinetics of crystal growth in the presence of surfactin molecules were also examined. As a result, we identified a new role of surfactin in the biosynthesis of porous calcite. The porous calcite formed and modified by biosurfactants can offer many new possibilities in biomedical and environmental applications.

2. Results and Discussion The calcium carbonate particles were synthesized in a spontaneous, rapid reaction in which calcium chloride and sodium carbonate were used as sources of calcium and carbonate ions, respectively. In the control test (without biosurfactant), turbidity appeared immediately after the Na2CO3 solution (pH 11.3) was poured into the CaCl2 solution without biosurfactant (pH 8.0), indicating the precipitation of CaCO3 particles. After 5 min of reaction, the sample vial was inserted into the measuring cell of the Turbiscan apparatus, and changes in the value of light passing through and reflected from the sample were recorded. Over time, the light transmittance value (T) increased significantly from 20 to approximately 75, reaching a plateau after approximately 15 min (Figure1A). A high value of T (%) indicates very unstable systems [17], in this case due to the formation of larger calcium carbonate particles that sediment. Similar results were observed where Ca2+ ions were in contact with biosurfactant particles for 24 h. However, in the samples with surfactin, the T (%) value increased much more slowly, reaching a value of approximately 70 after 1 h (see Figure1A). This is the result of the interaction of biomolecules during the process (nucleation and further growth of particles). To better understand the process, the ageing time was extended to 24 h, different surfactin concentrations were applied, and the polymorphic form of the crystals and morphology of the structures were analysed. Physicochemical and thermal properties of the biomineral were also measured. Int. J. Mol. Sci. 2020, 21, 5526 3 of 15 Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 3 of 15

Figure 1. (A) T (%) value changes during the time of reaction. (B) Zeta potential of calcium carbonate Figure 1. (A) T (%) value changes during the time of reaction. (B) Zeta potential of calcium carbonate precipitated at various surfactin concentrations after 24 h of ageing (NaCl concentration was 1 mM, precipitated at various surfactin concentrations after 24 h of ageing (NaCl concentration was 1 mM, standard deviation for zeta potential values was lower than 1 mV). standard deviation for zeta potential values was lower than 1 mV). 2.1. Particle Size Distribution and Zeta Potential of Calcium Carbonate after 24 H of Ageing To better understand the process, the ageing time was extended to 24 h, different surfactin The particle size distributions and the diameters of CaCO3 particles obtained in the control system concentrations were applied, and the polymorphic form of the crystals and morphology of the and with surfactin after 24 h of ageing are shown in Figure S1 and Table1, respectively. The addition of structuressurfactin towere the reactionanalysed. system Physicochemical resulted in a signiﬁcantand thermal particle properties size reduction. of the Thebiomineral median diameterwere also measured. (d50) of CaCO3 formed without the biosurfactant was 24.4 µm. Increasing the concentration of surfactin molecules up to 20 ppm decreased the value of d50 to 10.6 µm, and more particles with diameters lower 2.1.than Particle 5 µm Size were Distribution formed. Comparing and Zeta Potent the dataial ofobtained Calcium Carbonate with 10 and after 20 24 ppm H of surfactin, Ageing one can see

thatThe the particle difference size in thedistributi size ofons the resultingand the particlesdiameters was of notCaCO significant.3 particles This obtained means that in surfactinthe control systemeffectively and with blocked surfactin the growth after of24 CaCO h of ageing3 crystals, are even shown at 10 in ppm. Figure S1 and Table 1, respectively. The addition of surfactin to the reaction system resulted in a significant particle size reduction. The Table 1. Median diameter (d50), lower (d10) and upper (d90) deciles of calcium carbonate particles median diameter (d50) of CaCO3 formed without the biosurfactant was 24.4 μm. Increasing the obtained in the presence of biosurfactant after 24 h of ageing. (Standard deviation for d values was concentrationlower than of 1surfactinµ). molecules up to 20 ppm decreased the value of d50 to 10.6 μm, and more particles with diameters lower than 5 μm were formed. Comparing the data obtained with 10 and 20 ppm surfactin, one Surfactincan see that Concentration the difference [ppm] in thed10 size[µm] of thed resulting50 [µm] particlesd90 [µm] was not significant. This means that surfactin effectively0 blocked the growth 12.5 of CaCO 24.43 crystals, 44.6even at 10 ppm. 5 8.4 15.4 26.0 10 6.3 12.4 21.6 Table 1. Median diameter (d50), lower (d10) and upper (d90) deciles of calcium carbonate particles 20 5.0 10.6 19.6 obtained in the presence of biosurfactant after 24 h of ageing. (Standard deviation for d values was lower than 1 μ). The presence of surfactin molecules in the reaction system changes not only the size but also the Surfactin Concentration [ppm] d10 [µm] d50 [µm] d90 [µm] zeta potential value of CaCO3 particles. Based on the data presented in Figure1B, it can be seen that the zeta potential value of particles0 in the control sample12.5 changed from24.4 8.1 to 44.621.1 mV in a pH range − − from 7 to 9.8. The addition of surfactin5 at concentrations of8.4 10 and 2015.4 ppm increased26.0 the negative value of the zeta potential to 30 and 36.110 mV, respectively, at6.3 a pH of approximately12.4 21.610 0.3. The changes − − ± in zeta potential value were also20 observed by other researchers5.0 using10.6 polypeptides,19.6 fulvic acid or stearicThe acidpresence [18,19 of]. Fromsurfactin previous molecules data in in the the literature, reaction system it is known changes that biomolecules not only the cansize adsorb but also on the zetasolid potential faces formed value asof aCaCO result3 particles. of the crystallization Based on the process data presented and change in the Figure interfacial 1B, it energycan be [seen20,21 that]. theThis zeta may potential explain value the decrease of particles in particle in the size control in the sample presence changed of surfactin, from as − shown8.1 to in−21.1 Figure mV S1 in and a pH rangeTable from1. The 7 highto 9.8. negative The addition value of of zeta surfactin potential at ofconcentrations crystals obtained of 10 in theand presence 20 ppm ofincreased surfactin the negativeproves thatvalue biosurfactant of the zeta potential particles are to − present30 and on −36.1 the surfacemV, respectively, of particles. at a pH of approximately 10 ± 0.3. The changes in zeta potential value were also observed by other researchers using polypeptides, fulvic acid or stearic acid [18,19]. From previous data in the literature, it is known that biomolecules can adsorb on solid faces formed as a result of the crystallization process and change the interfacial energy [20,21]. This may explain the decrease in particle size in the presence of surfactin, as shown in Figure S1 and Table 1. The high negative value of zeta potential of crystals obtained in the presence of surfactin proves that biosurfactant particles are present on the surface of particles.

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2.2. XRD Analysis of Calcium Carbonate after 24 H of Ageing 2.2. XRD Analysis of Calcium Carbonate after 24 H of Ageing CrystallineCrystalline calcium calcium carbonate carbonate polymorphspolymorphs obtained obtained after after 24 h of24 ageingh of ageing were determined were determined by XRD by XRDanalysis. analysis. From From Figure Figure2, it can2, it be can seen be that seen in thethat control in the sample control (without sample surfactin) (without and surfactin) in the presence and in the presenceof 5 ppm of surfactin,5 ppm onlysurfactin, calcite only crystals calcite were obtained.crystals Thiswere means obtained. the complete This means transformation the complete of transformationthe metastable of phase, the metastable vaterite, into phase, a thermodynamically vaterite, into stablea thermodynamically phase, calcite, under stable these conditions.phase, calcite, underA furtherthese increaseconditions. in surfactin A further concentration increase upin tosurf 20actin ppm resultedconcentration in a mixture up to of calcite20 ppm and resulted vaterite in a mixtureparticles, of calcite which and was vaterite revealed particles, by the characteristic which was peaks revealed in Figure by the2. characteristic peaks in Figure 2.

FigureFigure 2. XRD 2. XRD patterns patterns of ofcalcium calcium carbonate carbonate precipitated precipitated at at di differentﬀerent surfactin surfactin concentrations concentrations after after 24 24

h of hageing. of ageing. The The concentration concentration of of CaCl CaCl22 andand Na Na2COCO3 was 55 mMmM (c-calcite, (c-calcite, v-vaterite). v-vaterite).

For 10 and 20 ppm surfactin in the reaction system, the vaterite content was found to be 1.4% and For 10 and 20 ppm surfactin in the reaction system, the vaterite content was found to be 1.4% and 14.9%, respectively (Table2). This means that the higher concentrations of surfactin molecules can 14.9%,effectively respectively delay the(Table transformation 2). This means of vaterite that tothe calcite. higher In concentrations all investigated samples,of surfactin a trace molecules amount can effectively delay the transformation of vaterite to calcite. In all investigated samples, a trace amount of Na2CO3 (0.1%, at 2Ө= 28◦) was found. On the basis of the calculated unit cell parameters of the of Napolymorphs2CO3 (0.1%, (Table at 22Ө), = a 28 possible˚) was incorporation found. On the of surfactinbasis of intothe calculated the crystal structureunit cell ofparameters calcite was of the polymorphsfound. For (Table that purpose, 2), a possible the relative incorporation changes (∆ l,of∆ Vsurfactin, %) in the into lengths the ofcrystal a and structure c edges and of volume calcite was found.in the For elemental that purpose, cell for the pure relative calcite changes without surfactin(Δl, ΔV, (%)l0, Vin0 )the and lengths for systems of a and with c varied edges amounts and volume of surfactin (ls, Vs) were calculated: ∆l = (ls l )/l 100. In all cases studied, a slight elongation of in the elemental cell for pure calcite without surfactin− 0 0· (l0, V0) and for systems with varied amounts of surfactin0.046–0.054% (ls, Vs) andwere 0–0.04% calculated: for the Δ al and= (l cs–l edges0)/l0·100 of the. In calcite all cases elemental studied, cell, respectively,a slight elongation was observed. of 0.046– 0.054%Nevertheless, and 0–0.04% the increasefor the a in and the edgec edges dimensions of the ca didlcite not elemental correspond cell, directly respectively, to an increment was observed. of surfactin concentration added to the reaction medium and seemed to become rather independent of its Nevertheless, the increase in the edge dimensions did not correspond directly to an increment of higher values (10 and 20 ppm). This small elongation of the edges, however, influences the volume of surfactin concentration added to the reaction medium and seemed to become rather independent of the calcite elemental cell (Table2). For the subsequent concentrations of 5, 10 and 20 ppm surfactin, its higherthe calculated values relative(10 and changes 20 ppm). in the This volume small of elongation calcite were of 0.085, the 0.142 edges, and however, 0.144%, respectively. influences the volumeSimilar of the results calcite were elemental observed cell for biogenic(Table 2). calcite For the and subsequent for calcite obtained concentrations in the presence of 5, 10 of and amino 20 ppm surfactin,acids and the proteins calculated [22,23 relative]. The changes changes in the in elementalthe volume cell parametersof calcite seemwere to 0.085, indicate 0.142 that surfactinand 0.144%, respectively.may induce Similar anisotropic results positive were lattice observed distortion for duebiogenic to the possiblecalcite and incorporation for calcite of biomoleculesobtained in the presenceinto the of crystalamino lattice. acids and This proteins phenomenon [22,23]. may The influence changes the crystalin the structureelemental of ce calcitell parameters in all studied seem to indicate that surfactin may induce anisotropic positive lattice distortion due to the possible incorporation of biomolecules into the crystal lattice. This phenomenon may influence the crystal structure of calcite in all studied systems. The effect of surfactin on the changes in the edge length and volume of the vaterite elemental cell is difficult to determine, as Table 2 shows, the increase in the values of particular parameters is in the range of standard deviation.

Int. J. Mol. Sci. 2020, 21, 5526 5 of 15 systems. The eﬀect of surfactin on the changes in the edge length and volume of the vaterite elemental cell is diﬃcult to determine, as Table2 shows, the increase in the values of particular parameters is in the range of standard deviation.

Table 2. Phase composition and the elemental cell parameters of the precipitated calcium carbonate calculated from the diﬀraction patterns. (Standard deviation for calcite or vaterite content was lower than 2%. Standard deviation for calcite cell parameters a, c and V were 0.003–0.008%, 0.001 0.006%, − 0.006 0.013%, respectively. Standard deviation for vaterite cell parameters a, c and V were 0.011–0.013%, − 0.025–0.029%, 0.23–0.34%, respectively).

Surfactin Concentration Calcite Vaterite [ppm] Content [%] Structural Parameters Content [%] Structural Parameters a = b = 4.9900(2) Å c = 17.0550(7) Å 0 100 -- α = β = 90◦, γ = 120◦ V = 367.776 (Å)3 a = b = 4.9923(3) Å c = 17.0537(14) Å 5 100 -- α = β = 90◦, γ = 120◦ V = 368.087 (Å)3 a = b = 4.9927(1) Å a = b = 4.1268(4) Å c = 17.0608(6) Å c = 8.4791(20) Å 10 98.6 1.4 α = β = 90◦, γ = 120◦ α = β = 90◦, γ = 120◦ V = 368.299 (Å)3 V = 125.057 (Å)3 a = b = 4.9926(2) Å a = b = 4.1299(5) Å c = 17.0619(9) Å c = 8.4699(25) Å 20 85.1 14.9 α = β = 90◦, γ = 120◦ α = β = 90◦, γ = 120◦ V = 368.308 (Å)3 V = 125.109 (Å)3

2.3. Morphology of Calcium Carbonate Crystals after 24 H of Ageing The morphology of the obtained calcium carbonate crystals was analysed using scanning electron microscopy,and the images are presented in Figure3. In Figure3A, it can be seen that without biomolecules, rhombohedral structures with a smooth surface and sharp edges characteristic of calcite were formed. The morphology changed when the biosurfactant was added to the reaction medium (Figure3B–F). The surface of the crystal formed in the presence of 5 ppm surfactin was deformed as some irregularities appeared (Figure3B). By analysing the images in Figure3, with higher biosurfactant concentrations, the presence of spherical structures characteristic of vaterite was observed, which was also conﬁrmed by XRD analysis (Figure2). However, the most interesting morphology was observed for calcite. Increased surfactin concentration resulted in a greater surface roughness of the calcite crystals, with spherical and oval cavities on the surface. Moreover, the edges of the crystals were not so sharp and were signiﬁcantly deformed (Figure3C–F). The deformation of calcite crystal microstructures after adding compounds of biological origin has been widely described in the literature [24–28]. Biomolecules, especially anionic molecules, can bind calcium ions in solution as well as on the surface of created crystals, changing the direction of growth or blocking the growth of the crystal surface [2]. Surfactin is a surfactant that reduces the surface tension of water to 28 mNm 1 at a critical micellization concentration (CMC) in − the range of 10–20 ppm, depending on pH and ionic strength [29]. Our previous studies showed that in a 5 mM aqueous solution of CaCl2 at pH 8, the CMC value for surfactin was 4.6 ppm [16]. Thus, the porosity of the calcite surface was a result of the formation of biosurfactant micelles in the reaction medium at surfactin concentrations above the CMC value. Similar results were obtained in the presence of nanosized spherules and worms of polymers and casein micelles [27,30]. Int. J. Mol. Sci. 2020, 21, 5526 6 of 15 Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 6 of 15

FigureFigure 3. SEM 3. SEM images images of of calcium calcium carbonate carbonate precipitation atat various various surfactin surfactin concentrations concentrations after after 24 h24 h ofof ageing.ageing. ( A()A control) control sample, sample, (B) 5 ppm,(B) 5 ( Cppm,,D) 10 ppm,(C,D) ( E10,F) 20ppm, ppm ( ofE,F surfactin.) 20 ppm The of concentration surfactin. The of

concentrationCaCl2 and Na of2 CaClCO3 2was and 5mM. Na2CO3 was 5mM.

In Inour our studies, studies, it was it was noted noted that that more more micelles micelles present present in the systemsystem leadlead to to more more porous porous calcite calcite crystals.crystals. Comparing Comparing these these results results with with previous previous results results [16], [16], it was observedobserved thatthat by by extending extending the the reactionreaction time time to to 24 h,h, porous porous crystals crystals are formedare form ated twice at thetwice biosurfactant the biosurfactant concentration. concentration. These structures These may have great potential for use as inorganic matrices or carriers of active compounds or nanoparticles. structures may have great potential for use as inorganic matrices or carriers of active compounds or It can be assumed that micelle concentration and ageing time play a crucial role in the formation of nanoparticles. It can be assumed that micelle concentration and ageing time play a crucial role in the porous calcite structures. formation of porous calcite structures. 2.4. BET Analysis of Calcium Carbonate after 24 H of Ageing 2.4. BET Analysis of Calcium Carbonate after 24 H of Ageing One of the most important parameters describing inorganic matrices is the value of specific surface areaOne (SSA) of the as well most as important the size and parameters volume of pores. describing The hysteresis inorganic loops matrices observed is inthe Figure value S2 of indicate specific surfacethe mesoporous area (SSA) structureas well as of the CaCO size3. Itand was volume observed of thatpores. particles The hysteresis obtained without loops observed biomolecules in Figure had 2 1 S2 aindicate small SSA the value mesoporous of 0.18 m structureg− (Table 3of). CaCO This is3. the It resultwas observed of rhombohedral that particles calcite formationobtained withwithout a biomoleculessmooth surface, had a as small shown SSA in Figurevalue3 ofA. 0.18 The m increase2g−1 (Table in surfactin 3). This concentration is the result of to 10rhombohedral ppm and 20 ppmcalcite 2 1 formationsignificantly with increased a smooth the surface, value of as SSA shown to 1.67 in andFigure 4.87 3A. m gThe− , respectively. increase in Asurfactin higher valueconcentration of the SSA to 10 arisesppm and from 20 the ppm roughness significantly of the calciteincreased surface the formedvalue of in SSA the presenceto 1.67 and of biosurfactant 4.87 m2g−1, respectively. (Figure3E,F). A higherA larger value specific of the surface SSA arises area was from obtained the roughness when a higher of the concentration calcite surface of biosurfactant formed in the was presence added to of biosurfactantthe reaction (Figure mixture. 3E–F). From A the larger data inspecific Table3 surf, it canace be area seen was that obtained the average when pore a higher diameter concentration increased of biosurfactantfrom 8.7 to 11.7 was nm added when surfactinto the reaction at higher mixture. concentrations From the was data added. in Table Moreover, 3, it can the be average seen that pore the averagevolume pore doubled diameter with increasingincreased biosurfactantfrom 8.7 to 11.7 concentration. nm when This surfactin may be at due higher to the concentrations formation of more was added.micelles Moreover, in the solution the average as well aspore the volume fact that atdoubled a 20 ppm with concentration increasing of biosurfactant biosurfactant, moreconcentration. vaterite Thiswas may formed be due (Table to the2). formation It is known of from more the micelles literature in that the vateritesolution has as awell higher as the SSA fact than that calcite at a [2031 ].ppm concentration of biosurfactant, more vaterite was formed (Table 2). It is known from the literature Table 3. The characteristic surface parameters of CaCO3 precipitate. that vaterite has a higher SSA than calcite [31]. Surfactin Concentration SSA Average Pores Volume Average Pores Diameter [ppm] 2 1 3 1 [nm] Table 3. The characteristic[m g− ] surface parameters[cm g− ] of CaCO3 precipitate. 0 0.18 0.05 - - ± Surfactin10 1.67 0.1 0.00513Average0.0002 Pores Average8.7 Pores1.0 SSA± ± ± Concentration20 4.87 0.09 0.0123Volume0.001 Diameter11.7 1.2 [m± 2g−1] ± ± [ppm] [cm3g−1] [nm] Comparing these0 data with our 0.18 previous± 0.05 results [16 -], one can say that - after 24 h of ageing, 0.00513 ± 8.7 ± 1.0 2 1 the specific surface area10 and average 1.67 pore ± volume0.1 slightly decreased from 7.11 to 4.87 m g− and from 0.0002 20 4.87 ± 0.09 0.0123 ± 0.001 11.7 ± 1.2 Comparing these data with our previous results [16], one can say that after 24 h of ageing, the specific surface area and average pore volume slightly decreased from 7.11 to 4.87 m2g−1 and from

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0.0202 to 0.0123 cm3g−1, respectively. This was related to the reduction of vaterite content and the 3 1 0.0202incorporation to 0.0123 of cm moreg− biosurfactant, respectively. molecules This was into related calcite to thecrystals. reduction of vaterite content and the incorporation of more biosurfactant molecules into calcite crystals. 2.5. Thermogravimetric Characteristics of Calcium Carbonate Structures 2.5. Thermogravimetric Characteristics of Calcium Carbonate Structures −1 Thermogravimetric analysis of calcium carbonate samples with 5 Kmin1 heating rate was also Thermogravimetric analysis of calcium carbonate samples with 5 Kmin− heating rate was also performed. To show the effect of surfactin on the thermal behaviour of CaCO3, samples with the performed. To show the effect of surfactin on the thermal behaviour of CaCO , samples with the most most differing surfactin content were selected, namely, without and with 203 ppm surfactin. It was differing surfactin content were selected, namely, without and with 20 ppm surfactin. It was observed observed that in both samples, with and without biosurfactant, the main degradation process started that in both samples, with and without biosurfactant, the main degradation process started above 690 C, above 690°C, which was related to the conversion of calcite into calcium oxide (Figure 4A). Despite◦ which was related to the conversion of calcite into calcium oxide (Figure4A). Despite the fact that two the fact that two additional weight losses were detected at lower temperatures (Figure 4A). The first additional weight losses were detected at lower temperatures (Figure4A). The first loss had no specified loss had no specified maximum on the DTG curve. The second loss was observed only for the maximum on the DTG curve. The second loss was observed only for the sample with surfactin and sample with surfactin and had a broad DTG peak with maximum about 510°C (Figure 4B). From the hadliterature, a broad it DTG is known peak that with the maximum first weight about loss 510 is ◦relatedC (Figure to 4theB). evaporation From the literature, of moisture it is contained known that in the first weight loss is related to the evaporation of moisture contained in the samples, and the second the samples, and the second weight loss is related to the evaporation of water bound in the CaCO3 weightstructure loss and/or is related transformation to the evaporation of vaterite of water to calc boundite [32,33]. in the To CaCO compare3 structure the chemical and/or transformationcomposition of ofthe vaterite sample to with calcite and [32 ,without33]. To comparesurfactin, the the chemical weight compositionloss observed of during the sample this withtwo recesses and without was surfactin,determined. the weightThe temperature loss observed ranges during was thisselected two recessesaccording was to determined.DTG results. TheIt was temperature shown that ranges in the wasfirst selected range (25–400 according °C), to the DTG weight results. ofIt the was materi shownal thatwith in surfactin the first rangedecreased (25–400 by ◦0.95%.C), the Continued weight of theheating material resulted with surfactinin a further decreased decrease by in 0.95%. material Continued weight to heating the same resulted value. in For a further a sample decrease without in materialsurfactin, weight it was to not the samepossible value. to divide For a sample this te withoutmperature surfactin, region itfor was two not processes, possible toas divide any DTG this temperaturemaximum regionin this forarea two was processes, observed. as any For DTG this maximum reason, the in thisweight area wasloss observed.of this material For this reason,in one thetemperature weight loss range of this from material 25 to in 566 one °C temperature was measur rangeed. Its from value 25 was to 566 approximately◦C was measured. 0.71%. Its The value higher was approximatelyweight loss value 0.71%. in The this higher temperature weight loss range value (25– in566 this °C) temperature for the sample range (25–566 with surfactin◦C) for the could sample be withconnected surfactin with could better be connected moisture with adsorption better moisture and surfactin adsorption molecule and surfactin degradation. molecule degradation.Also, these Also,differences these differences were visible were in the visible total inweight the total loss weightof the samples, loss of the which samples, was higher which for was the higher sample for with the samplesurfactin with by surfactin approximately by approximately 1%. The maximum 1%. The maximumDTG associated DTG associatedwith calcite with conversion calcite conversion was also washigher also for higher this sample for this samplewith surfactin, with surfactin, which could which demonstrate could demonstrate obstruction obstruction of this process of this processby the use by theof usebiosurfactant of biosurfactant (Figure (Figure 4 and 4Table and Table4). 4).

FigureFigure 4. 4.TG TG and and DTG DTG curves curves of of the the calcium calcium carbonate carbonate structures structures obtained obtained without without and and with with 20 20 ppm ppm of biosurfactant.of biosurfactant. Time Time of reaction of reaction was was 24 h. 24 The h. concentrationThe concentration of CaCl of CaCl2 and2 Naand2 CONa32COwas3 was 5 mM. 5 mM. (A) TG;(A) (BTG;) DTG. (B) DTG.

Table 4. Thermogravimetry results of the samples (∆m–weight loss, T–maximum on the DTG curve, Table 4. Thermogravimetry results of the samples (Δm–weight loss, T–maximum on the DTG curve, onset-temperature- of main degradation start). onset-temperature- of main degradation start). Surfactin Concentration Surfactin Concentration∆m 1 (25–400 ◦C) [%] ∆m2 (400–566 ◦C) [%] T1 [◦C] TT21[ ◦C] T2 ∆ mtotal∆mtotal[%] [ppm] ∆m1 (25–400 °C) [%] ∆m2 (400–566 °C) [%] [ppm] [°C] [°C] [%] 0 0.71 - 717 43.69 200 0.950.71 0.95 510- 724 717 44.7143.69 20 0.95 0.95 510 724 44.71

Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 8 of 15

In addition, model-free kinetics were used to determine activation energy values as a function of material conversion [34]. To achieve this, both samples were analysed by the thermogravimetric method with four different heating rates (5, 10, 15, and 20 Kmin−1) and presented as a conversion curve (Figure S3). The activation energy value was calculated as follows (1):

− = () (1)

where, α is conversion, t—time, A—frequency factor, R—gas constant, Ea—activation energy, T—temperature. A comparison of the obtained results for samples without and with 20 ppm surfactin showed that they differed from each other. For a material without surfactin, the Ea value was almost constant over the entire conversion range (~160 kJmol−1). This means that only one process was observed during thermal degradation of the control sample—calcite conversion to calcium oxide. While analysing the sample with surfactin, it was observed that the first Ea value was very low at the beginning of the experiment. Then, it increased rapidly to 250 kJmol−1. Further activation energy decreased to a similar value as for the sample without surfactin. Three processes can be defined during the thermal degradation of this sample. The first was assumed to be moisture release, the second was the conversion of vaterite to calcite or bonded water evaporation, and the third was the decomposition of calcite to CaO (Figure 5A). Int. J. Mol. Sci. 2020, 21, 5526 8 of 15 During the TG results interpretation, the thesis was put forward that the first degradation process was related to water evaporation and the second one to bound water evaporation and/or vateriteIn addition, transformation model-free to calcite. kinetics To verify were this used mass to determine spectrometry, activation analysis energy was performed. values asa In function QMS ofstudies, material changes conversion in ionic [34 ].current To achieve as a function this, both of samples temperature were were analysed investigated by the thermogravimetric for specific m/z values for water (18) and carbon dioxide (44). The obtained curv1 es demonstrated that in both method with four diﬀerent heating rates (5, 10, 15, and 20 Kmin− ) and presented as a conversion curvesamples (Figure (with S3). and The without activation surfactin), energy water value was was rele calculatedased during as follows the first (1): thermal degradation (till about 400°C). These results excluded the thesis about the evaporation of bound water from the samples at approximately 510°C (Figured α5B). For thisE reason,a it was concluded that this process is = Aexp − f (α) (1) most likely related to the transformationdt of vateriteRT to calcite. Such broad temperatures range for water release from the samples could indicate that their molecules are not only physically but also where, α is conversion, t—time, A—frequency factor, R—gas constant, E —activation energy, T— chemically adsorbed on the material surface, for which desorption higher temperaturesa are needed temperature. A comparison of the obtained results for samples without and with 20 ppm surfactin [35,36]. The changes in m/z 44 related to CO2 confirmed that this gas is evolving during the thermal showed that they differed from each other. For a material without surfactin, the Ea value was almost degradation of CaCO3. It can also be seen that this process occurs in a higher temperature range for 1 constantthe sample over with the entiresurfactin conversion (Figure 5B). range This (~160 is consis kJmoltent− ). with This meansthe results that of only the one TG processanalysis was (Figures observed 4 duringand 5A). thermal Additionally, degradation another of the important control sample—calcite conclusion can conversion be drawn tofrom calcium these oxide. studies. While It has analysing been E thenoted sample that withat temperatures surfactin, it below was observed 200 °C, thatCO2 was the firstderiveda value from was the verysample low with at the surfactin, beginning which of the 1 experiment.was not indicated Then, it increasedfor the reference rapidly tosample. 250 kJmol This− .confirms Further activationthe incorporation energy decreased or adsorption to a similar of valuesurfactin as for on the the sample CaCO without3 surface surfactin. (Figure Three5B). All processes observations can be definedwere consistent during the with thermal the XRD degradation data of(Table this sample. 2) and TheSEM first images was assumed(Figure 3), to bewhich moisture reveal release,ed vaterite the second in the was sample the conversion precipitated of vateritewith a to calcitesurfactin or bonded concentration waterevaporation, of 20 ppm. and the third was the decomposition of calcite to CaO (Figure5A).

Figure 5. (A) Ea changes as a function of materials conversion. (B) QMID (Quasi Multiple Ion Detection) value for m/z 18 (water) and 44 (CO2).

During the TG results interpretation, the thesis was put forward that the first degradation process was related to water evaporation and the second one to bound water evaporation and/or vaterite transformation to calcite. To verify this mass spectrometry, analysis was performed. In QMS studies, changes in ionic current as a function of temperature were investigated for specific m/z values for water (18) and carbon dioxide (44). The obtained curves demonstrated that in both samples (with and without surfactin), water was released during the first thermal degradation (till about 400 ◦C). These results excluded the thesis about the evaporation of bound water from the samples at approximately 510 ◦C (Figure5B). For this reason, it was concluded that this process is most likely related to the transformation of vaterite to calcite. Such broad temperatures range for water release from the samples could indicate that their molecules are not only physically but also chemically adsorbed on the material surface, for which desorption higher temperatures are needed [35,36]. The changes in m/z 44 related to CO2 confirmed that this gas is evolving during the thermal degradation of CaCO3. It can also be seen that this process occurs in a higher temperature range for the sample with surfactin (Figure5B). This is consistent with the results of the TG analysis (Figures4 and5A). Additionally, another important conclusion can be drawn from these studies. It has been noted that at temperatures below 200 ◦C, CO2 was derived from the sample with surfactin, which was not indicated for the reference sample. This confirms the incorporation or adsorption of surfactin on the CaCO3 surface Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 9 of 15 Int. J. Mol. Sci. 2020, 21, 5526 9 of 15

Figure 5. (A) Ea changes as a function of materials conversion. (B) QMID (Quasi Multiple Ion

(FigureDetection)5B). All value observations for m/z 18 were(water) consistent and 44 (CO with2). the XRD data (Table2) and SEM images (Figure3), which revealed vaterite in the sample precipitated with a surfactin concentration of 20 ppm. 2.6. Effect of Ageing Time on Crystal Growth 2.6. Effect of Ageing Time on Crystal Growth To describe more in detail how the porous structures of calcium carbonate were formed, as shown Toin Figure describe 3, more the effect in detail of howageing the time porous on structures the crystalline of calcium structure carbonate and weremorphology formed, asof showncrystals in Figure3, the e ffect of ageing time on the crystalline structure and morphology of crystals was was studied. The experiments were carried out in a reaction system containing 10 ppm surfactin. It studied. The experiments were carried out in a reaction system containing 10 ppm surfactin. It was was observed that in the system without the biosurfactant, pure calcite was precipitated in all observed that in the system without the biosurfactant, pure calcite was precipitated in all studied studied processes during the time periods (Figure S4A). The presence of biosurfactant molecules led processes during the time periods (Figure S4A). The presence of biosurfactant molecules led to the to the formation of a mixture of calcite and vaterite even after 24 h (Figure S4B). The mass fraction of formation of a mixture of calcite and vaterite even after 24 h (Figure S4B). The mass fraction of the theresulting resulting phases phases is givenis given in Table in 5Table. As expected,5. As ex thepected, amount the of amount vaterite of decreased vaterite with decreased extending with extendingreaction reaction time. The time. highest The concentration highest concentration of vaterite of was vaterite 40.5% was after 40.5% 30 min after of ageing, 30 min and of theageing, lowest and the(1.4%) lowest was (1.4%) after was 24 h after of ageing. 24 h of As ageing. shown As in shown the control in the sample, control pure sample, calcite pure was calcite formed was just formed after just30 after min 30 of ageingmin of (Figureageing S4A).(Figure Therefore, S4A). Therefore, this proves this that proves biosurfactant that biosurfactant molecules slow molecules down theslow downconversion the conversion of vaterite of intovaterite calcite. into When calcite. analysing When analysing the SEM images the SEM (Figure images6A–F), (Figure it was 6A–F), noted thatit was notedmore that surface more irregularities surface irregularities appeared during appeared longer duri reactionng longer times. reaction More cavities times. were More visible, cavities and thewere visible,edges and of calcite the edges were of significantly calcite were deformed, significantl showingy deformed, a multilayered showing structurea multilayered after 24 structure h of ageing. after 24 Thirtyh of ageing. minutes Thirty of ageing minutes led of to ageing the formation led to th ofe calciteformation with of slightly calcite deformed with slightly edges. deformed Few cavities edges. Fewwere cavities visible were on the visible calcite on surface. the calcite This surface. may be dueThis to may the largebe due amount to the of large vaterite amount that was of vaterite formed inthat wasthis formed process, in whichthis process, was 40.5% which (Table was5). 40.5% (Table 5).

FigureFigure 6. 6.SEMSEM images images of of calcium carbonatecarbonate precipitated precipitated in 10in ppm 10 ppm of surfactin. of surfactin. (A–C) ( 30A–C min) 30 of aging,min of 2 2 3 aging,(D) 1(D h) of 1 aging,h of aging, (E) 3 h(E of) 3 aging, h of aging, (F) 24 h(F of) 24 aging. h of aging. The concentration The concentration of CaCl2 ofand CaCl Na2 COand3 NawasCO 5 mM. was 5 mM. Table 5. Phase composition of precipitated calcium carbonate calculated from the diﬀraction patterns. (Standard deviation for calcite/vaterite content was lower than 1%). Surfactin concentration was 10 ppm. Table 5. Phase composition of precipitated calcium carbonate calculated from the diffraction pH was 8. patterns. (Standard deviation for calcite/vaterite content was lower than 1%). Surfactin concentration was 10 ppm. pH was 8. Time [hours] Calcite [%] Vaterite [%]

Time0.5 59.6 40.5 1Calcite 67.8 [%] Vaterite 32.2 [%] [hours]3 90.5 9.5 0.524 59.6 98.6 40.5 1.4 1 67.8 32.2 Additionally, the changes in the3 average90.5 crystallite size 9.5 (D ) and microstrain (ε) of vaterite and calcite in samples as a function of time24 were calculated98.6 according 1.4 to the Williamson-Hall method (W-H method)Additionally, [37]. It wasthe changes observed in that the theaverage size of crystallite calcite crystallite size (D) in and both microstrain samples increased (ε) of vaterite with the and calcite in samples as a function of time were calculated according to the Williamson-Hall method (W-H method) [37]. It was observed that the size of calcite crystallite in both samples increased with the progress of the reaction. The values of D and ɛ in the control sample were higher than those in

Int. J. Mol. Sci. 2020, 21, 5526 10 of 15 Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 10 of 15 theprogress sample of thewith reaction. surfactin. The valuesThe crystallite of D and ε insize the in control the control sample weresample higher increased than those sharply in the sampleafter 24 h (Figurewith surfactin. 7A), which The crystallite is characteristic size in the for control the samplecrystallization increased process. sharply afterBy analysing 24 h (Figure the7A), microstrain which results,is characteristic it was observed for the crystallization that their values process. were By at analysing similar levels the microstrain for both results,samples. it wasThe observedonly change wasthat theirnoticed values in werethe control at similar sample levels after for both 24 samples.h (Figure The 7B). only This change shows was that noticed surfactin in the controlmolecules influencedsample after the 24 crystallization h (Figure7B). This process shows of that calcium surfactin carbonate. molecules An influenced increase thein the crystallization size of crystallites process and microstrainsof calcium carbonate. of calcite An results increase from in the the size clasical of crystallites ion by and ion microstrains attachment of calciteas well results as the from oriented the aggregation.clasical ion by The ion increase attachment in the as wellmicrostrain as the oriented is due aggregation.to formation Theof crystal-crystal increase in the interface microstrain [38]. is The lowerdue to value formation of D of and crystal-crystal ɛ after 24 h interface results [38from]. The the lower incorporation value of D ofand organicεafter 24additivities h results frominto the crystalsthe incorporation as well as ofdecreasing organic additivities the crystals into boundary the crystals density as well [39]. as decreasing the crystals boundary densityCrystallites [39]. of vaterite were found only in the sample with surfactin. Their size and microstrain valuesCrystallites decreased of as vaterite a function were of found time onlydue to in thedissolution/recrystallization sample with surfactin. Their reactions size and (Figure microstrain 7C–D). values decreased as a function of time due to dissolution/recrystallization reactions (Figure7C,D).

Figure 7. Changes in crystallite size (D) and microstrains (ε) of CaCO crystals obtained with and Figure 7. Changes in crystallite size (D) and microstrains (ε) of CaCO3 3 crystals obtained with and without surfactin. Calcite (A,B); vaterite (C,D). without surfactin. Calcite (A,B); vaterite (C,D).

To understand the observed phenomenon, the mechanism of CaCO3 crystallization in the presence To understand the observed phenomenon, the mechanism of CaCO2+ 3 crystallization2- in the of surfactin should be analysed. According to the literature, the mixing of Ca with CO3 ions results 2+ 2- presencein the formation of surfactin of nuclei should and thebe analysed. synthesis of A amorphousccording to calcium the literature, carbonate the (ACC) mixing nanoparticles of Ca with [28 CO]. 3 ionsThe precursorresults inphase the formation is highlyhydrated of nuclei and and the the most synthesis unstable of phase amorphous of calcium calcium carbonate. carbonate The ACC (ACC) nanoparticlesnanoparticles are[28]. equilibrated The precursor in solution phase is quickly highly (within hydrated seconds) and the and most transformed unstable intophase a calcium of calcium carbonate.carbonate polymorphThe ACC (vaterite).nanoparticles The transformationare equilibrated of ACCin solution to vaterite quickly can occur (within through seconds) a direct and transformedsolid-state transformation, into a calcium and carbonate it is experimentally polymorph difficult (vaterite). to capture The [ 40transformation]. ACC nanoparticles of ACC aggregate to vaterite caninto occur larger through particles, a and direct dehydration solid-state of thetransformation, precursor phase and takes it is place.experimentally This leads todifficult the formation to capture [40].of spherical ACC nanoparticles vaterite particles, aggregate a metastable into larger phase. pa Vateriterticles, is and quickly dehydration combined of in surface-mediatedthe precursor phase takesdissolution-reprecipitation place. This leads to the reactions, formation leading of spherical to the formation vaterite of particles, pure rhombohedral a metastable calcite. phase. This Vaterite leads is quicklyto a reduction combined in the numberin surface-mediated of crystals and andissolution-reprecipitation increase in particle diameter, asreactions, shown in leading Figure1A. to the formationBased of on pure the obtained rhombohedral results and calcite. literature This data,leads it to can a bereduction assumed in that the surfactin number molecules of crystals act and in an increasethe biomineralization in particle diameter, of calcium as carbonateshown in atFigure two di 1A.ﬀerent stages, which are mutually dependent: Based on the obtained results and literature data, it can be assumed that surfactin molecules act in the biomineralization of calcium carbonate at two different stages, which are mutually dependent: (a) in solution during the nucleation stage (formation of Ca-surfactin complexes), (b) on the surface of formed CaCO3 crystallites.

Int. J. Mol. Sci. 2020, 21, 5526 11 of 15

(a) in solution during the nucleation stage (formation of Ca-surfactin complexes),

(b) on the surface of formed CaCO3 crystallites.

Surfactin is a lipopeptide biosurfactant composed of a cyclic ring of amino acids linked to hydroxylic fatty acids by a lactone bond [41,42]. The cyclic peptide part contains two hydrophilic residues of Glu and Asp acid because of free groups of COO− at a pH above pKa = 5.8 [43,44]. These two negatively charged groups in the surfactin molecule are considered to form complexes with Ca2+ ions at pH 8 and enhanced the intermicellar aggregates formation in β-sheet conformation, as discussed in detail in the literature [45,46]. This can slightly reduce the concentration of free Ca2+ during the nucleation stage (Figure8), because in the system an excess of calcium and carbonate ions compared to surfactin molecules was used. Moreover, the life span of this bond is short, and when carbonate ions are 2+ 2 added, the released and free Ca ions interact with CO3 − ions [16] and form hydrated nuclei and vaterite-calcite mixture in the subsequent step. The free surfactin molecules can be adsorbed onto the selected crystals phase. It is known from the literature that during crystallization, vaterite particles may have a positive charge on the surface [28]. Surfactin molecules can adsorb on the vaterite surface via electrostatic interactions as well as via hydrogen bonds between protonated amino groups and oxygen in the carbonate group on the crystal surface [47,48]. The presence of surfactin molecules on the solid/liquid interface changed the solubility of vaterite [18]. As a result, further conversion of crystals into calcite slowed down (Table5), followed by gradual changes in morphology (Figure6), crystallite size (Figure7C) and microstrain (Figure7D). Additionally, changes in the T(%) value (Figure1A) prove that the small particles are dispersed in the system. This inhibiting effect of surfactin intensifies as the biomolecule concentration increases. More vaterite remained in the system after 24 h of ageing when more biosurfactant was added (Table2). A similar mechanism was proposed by Wu et al. (2018) [ 49], where citrate anions were used as a modifying agent. However, in this case, the surface morphology of calcite was significantly deformed (Figure3D,E and Figure6F). Some slight cavities appeared on the surface of calcite crystals obtained in the presence of 10 ppm surfactin after 24 h of ageing. Moreover, the edges of calcite particles obtained after 24 h were blunted, and micelles embedded in crystals were observed (Figure S5.), which was also confirmed by the lattice parameter values (Table2), where positive distortion was observed. Additionally, TGA and QMS analyses (Figure5B) prove the occlusion of the surfactin molecules. Our results confirm the hypothesis that the deformation of the crystals results from the existence of micelles in the reaction medium. Prolonging the time of reaction up to 24 h, the percentage of vaterite in the sample decreased and more calcite is formed (Table5) due to the dissolution of the metastable phase. The released inorganic ions as well as the aggregates of micelles may accumulate (according to the diffusion gradient) near the surface of large crystals formed earlier (Figure8). The ions interact with the external functional group of the surfactin, and the new calcite grains are crystallized. The clusters formed are attached to the calcite surface, and finally, mesoporous calcite crystals with a small diameter and larger specific surface area are produced after 24 h of ageing. The mechanism of calcite formation in the presence of surfactin is similar to that which occurs in nature [50]. The presence of micelles at the surface of crystals formed can block further growth due to the decrease in the zeta potential value of crystals formed. This has reduced crystal aggregation via electrostatic repulsion between the crystals. This was confirmed by zeta potential and T changes (Figure1A,B), SEM images (Figure6), crystallite size (Figure7A), and the size distribution of CaCO 3 particles (Figure S1). The higher the concentration of biosurfactant in the solution, the more micelles were incorporated on the crystal surface, as confirmed by SEM images (Figure3), the zeta potential values (Figure2) and TGA (Figure5B). Additionally, the size of pores (cavities) on the surface is close to the size of a single micelle of biosurfactant, ca. 10 nm [51]. It is characteristic that the surface roughness increases with time (Figure6). Additionally, the size of crystallites in the sample with surfactin after 24 h of ageing is significantly lower than in the control sample (Figure7A). This results from oval and spherical depletions on the calcite crystal surface [52]. On the calcite surface, the edges are more rounded and truncated, indicating the presence of micelles on the edges of the crystals. The surface of calcite was less deformed when there was more vaterite in the system. Since calcite has a negative Int. J. Mol. Sci. 2020, 21, 5526 12 of 15

Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 12 of 15 zeta potential value (Figure1B), negatively charged micelles prefer to adsorb on a positively charged vateritecalcite surface.was produced This explains in the whypresence more of vaterite surfactin than molecules. calcite was Thus, produced as long in the as presencevaterite particles of surfactin are molecules.present in Thus,the system, as long the as calcite vaterite surface particles will are not present be significantly in the system, distorted, the calcite as shown surface in Figure will not 6. be signiﬁcantly distorted, as shown in Figure6.

Figure 8. Mechanism of mesoporous calcite formation. Figure 8. Mechanism of mesoporous calcite formation.

3.3. Materials Materials and and Methods Methods CalciumCalcium chloride chloride from from Fluka Fluka (Bucharest, (Bucharest, Romania) Romania) and and sodium sodium carbonate carbonate (p.a.) (p.a.) from from Avantor Avantor PerformancePerformance Materials Materials Poland Poland S.A. S.A. (Gliwice, (Gliwice, Poland) Poland) were were used used as as reactants. reactants. Surfactin Surfactin (purity (purity 91%) 91%) waswas purchased purchased from from Lipofabrik Lipofabrik (France). (France). The The procedure procedure of of calcium calcium carbonate carbonate microstructure microstructure synthesis synthesis waswas described described in in Bastrzyk Bastrzyk et et al. al. (2019) (2019) [ 16[16].]. First, First, the the calcium calcium chloride chloride solution solution was was conditioned conditioned with with surfactinsurfactin at at various various concentrationsconcentrations at pH pH 8. 8. Then, Then, the the CaCO CaCO3 crystals3 crystals were were obtained obtained by bymixing mixing two two 0.01 0.01M equimolar M equimolar solutions solutions of CaCl of CaCl2 (without2 (without and with and withthe biosurfactant) the biosurfactant) and Na and2CO Na3 at2CO constant3 at constant stirring stirringintensity intensity using a usingmagnetic a magnetic stirrer. After stirrer. adding After Na adding2CO3, the Na pH2CO of3 the, the reaction pH of medium the reaction was mediumincreased wasto 10.5. increased The reaction to 10.5. times The were reaction 30 min, times 1 h, were 3 h, 30 and min, 24 h. 1 h,After 3 h, the and mixing 24 h. time, After the the precipitate mixing time, was theseparated, precipitate dried was and separated, characterized. dried and The characterized. concentrations The of concentrationsbiosurfactant were of biosurfactant 5, 10 and 20 were ppm. 5, The 10 andcontrol 20 ppm. sample The was control synthesized sample without was synthesized the biosurfa withoutctant. theAll biosurfactant.experiments were All experimentscarried out at were room carriedtemperature out at (25°C), room temperature and three separate (25 ◦C), experiments and three separate were performed. experiments were performed. TheThe precipitate precipitate was was characterized characterized after dryingafter drying using scanningusing scanning electron microscopyelectron microscopy (SEM), Brunauer- (SEM), Emmett-TellerBrunauer-Emmett (BET)-Teller analysis (BET) and thermogravimetryanalysis and thermogravim (TG) coupledetry with (TG) gas masscoupled spectrometry with gas (QMS) mass analysis.spectrometry The particle (QMS) size analysis. distribution The andparticle zeta potentialsize distribution value of CaCO and 3zetawere potential also measured. value of All CaCO procedures3 were foralso characterizing measured. the All CaCO procedures3 particles arefor includedcharacterizing in the Supplementary the CaCO3 Materialsparticles (pleaseare included see Section in S1.1). the SupplementaryAll reagents wereMaterials used without(please see further Section purification. S1.1). Water was provided from a WG-LHP purification 1 systemAll (WIGO, reagents Poland) were withused a conductivitywithout further of 0.055 purificµScmation.− . Water was provided from a WG-LHP purification system (WIGO, Poland) with a conductivity of 0.055 μScm−1. 4. Conclusions 4. Conclusions Based on the research carried out, it can be concluded that surfactin can effectively affect the properties of precipitatedBased on calciumthe research carbonate. carried Light out, transmittanceit can be conc valueludedchanges that surfactin confirmed can effectively the hypothesis affect that the biosurfactantproperties of molecules precipitated delayed calcium the growth carbonate. of calcium Light carbonate transmittance crystals. Thevalue presence changes of biosurfactant confirmed inthe thehypothesis reaction system that biosurfactant significantly changedmolecules the precipitatedelayed the morphology, growth of especially calcium forcarbonate calcite. Thecrystals. structure The ofpresence the crystals of biosurfactant was deformed, in andthe afterreaction 24 hsystem of reaction, significantly mesoporous changed calcium the carbonateprecipitate was morphology, obtained atespecially a low biosurfactant for calcite. concentration The structure (10 of ppm). the cr Theystals specific was surfacedeformed, area ofand the after porous 24 calciteh of reaction, was 10 timesmesoporous larger than calcium that of carbonate the control was sample obtained without at surfactin. a low biosurfactant The porous structureconcentration of calcium (10 ppm). carbonate The specific surface area of the porous calcite was 10 times larger than that of the control sample without surfactin. The porous structure of calcium carbonate results from the adsorption and occlusion of micelles in the crystals during growth. The pore size was similar to the size of a single micelle,

Int. J. Mol. Sci. 2020, 21, 5526 13 of 15 results from the adsorption and occlusion of micelles in the crystals during growth. The pore size was similar to the size of a single micelle, indicating the adsorption of micelles on calcite crystals during crystallization, which was proven by zeta potential measurements, lattice parameter calculations and TGA. The incorporation of micelles did not change the thermal properties of the particles. Moreover, the biomolecules can be removed from the crystals by heating at temperatures below 200 ◦C. These studies provided valuable information on the formation of inorganic matrices with unique morphologies and surface properties, which can be of great benefit for biomedical and environmental applications. In addition, this is the ﬁrst report on the study of calcite growth over time in a micelle system of an environmentally friendly surfactant.

Supplementary Materials: Supplementary materials can be found at http://www.mdpi.com/1422-0067/21/15/ 5526/s1. Author Contributions: Conceptualization, A.B.; methodology, A.B. and M.F.-T.; investigation, A.B. and M.F.-T.; data curation, A.B., M.F.-T., H.M. and I.P.; writing-original draft preparation, A.B. and M.F.-T.; writing-review and editing, A.B., M.F.-T., H.M., I.P. and G.P.; visualization, A.B. and M.F.-T.; supervision, G.P.; project administration, A.B.; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Science Centre of Poland grant no. 2014/15/D/ST8/00544. The APC was funded by subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Science and Technology. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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