Organic-nanoclay composite materials as removal agents for environmental decontamination

Here we overview the recent advances in the fabrication of sustainable composite nanomaterials with decontamination capacity towards inorganic and organic pollutants. In this regards, we present the development of hybrid systems based on clay nanoparticles with different shapes (such as kaolinite nanosheets and halloysite nanotubes) and organic molecules (biopolymers, surfactants, cucurbituril) as efficient removal agents for both aliphatic and aromatic hydrocarbons. Due to their high specific surface area, clay nanoparticles have been successfully employed as fillers for composite membranes with excellent filtration capacity. The preparation of composite gel beads based on biopolymers (alginate and pectin) and halloysite nanotubes has been discussed and their adsorption capacities towards both heavy metals and organic dyes have been highlighted. We describe the successful preparation of kaolinite/graphene composites as well as tubular inorganic micelles obtained by the select functionalization of the halloysite cavity with anionic surfactants. Finally, recent research on Pickering emulsions (for oil spill remediation) and bioremediation technologies has been discussed.


Introduction
Nowadays, environmental pollution is one of the biggest world problems. Contaminants have been present in the environment since time immemorial: including elements of volcanic dust, comets and cosmic dust, which account for about 100 tons of organic dust per day. 1 Currently, the spectrum of pollutants has expanded signicantly including heavy metals, 2 hydrocarbons (aliphatic, aromatic and polycyclic aromatic hydrocarbons), 3 BTEX (benzene, toluene, ethylbenzene and xylenes), 4 chlorinated hydrocarbons, 5 trichloroethylene (TCE) and perchloroethylene, 6 nitroaromatic compounds, 7 organophosphorus compounds 8 and pesticides. 9 Generally, pollutants enter wastewater and then are present in rain, fog and snow. 10 Oil pollution is directly related to human factors, such as the deliberate discharge of waste. 11 Nanotechnology products are considered as very effective tools for environmental clean-up. The use of many types of nanoparticles and methods for cleaning natural resources and improving the quality of life of the population is widely described. For example, nanolter membranes are widely used to remove dissolved salts and micropollutants, as well as water soening and wastewater treatment. 12 The use of nanomaterials for the treatment of water resources and soils has been demonstrated for chitosan and silver nanoparticles 13,14 as well as for carbon nanomaterials. 15 The use of zero-valence iron as a reducing agent for cleaning contaminated sites from polychlorinated biphenyls, pesticides, herbicides, aromatic hydrocarbons and metals has been reported. [16][17][18][19] Nanomaterialsbased remediation methods include the use of nanomaterials for detoxication and transformation of pollutants. In this regards, titanium oxide can be used in the photo-oxidation of organic pollutants. 20 Biodegradation, sorption, hydrolysis, photolysis and microltration, UV irradiation represent alternative strategies for the pollutants removal. 21,22 Recently, various types of nanoclays have become important targets for applications in environmental industries and bioremediation. 23 For example, clay nanoparticles may absorb various pollutants, including organic (atrazine, triuralin, parathion and malathion) and inorganic compounds (for example, metals: copper, zinc, cadmium and mercury, etc.) from soil and wastewater. In addition, industrial and biomedical use of nanoclays is increasing: planar nanoclays bentonite, 24 montmorillonite, 25 kaolin 26 and tubular mineral halloysite 27 have been used as nanoscale llers for the manufacture of polymer composites, 28 anti-corrosion and ame retardant coatings. 29,30 The impressive effect of nanoclay particles to improve the structural 31 and functional properties of biomaterials, along with their availability and low-cost production, suggests the nanoclay's use will constantly increase 32 Sorbents based on clay minerals have unique properties, such as high specic surface area, reusability, low cost, and ubiquity in the natural environment. 33 There are reports on the use of bentonite for the sorption of amoxicillin antibiotic from liquid suspensions, 34 and the adsorption of trimethoprim using montmorillonite clay. 35 In addition, halloysite nanotubes are considered as an adsorbent for purifying water, for example, from heavy metals, 36 dyes 37 and aromatic pollutants. [38][39][40] Recent studies provide an opportunity to improve clay minerals, allowing to increase their sorption capacity (from 67.0% to 98.9%) and selectivity for specic metals, thereby opening up a new sphere for their use. 41 Researchers proposed clay/Fe 3 O 4 composites as adsorbents to remove heavy metals and dye molecules from modelled wastewater through magnetic separation. [42][43][44] Analysis of recent literature sources has shown that textile wastewaters are considered to be the most polluting among all other industrial effluents due to their complex composition 45 and the possibility of using natural minerals to clean them should be carefully investigated. Moreover, there are several advantages concerning the possibility to recover and to reuse clays aer the decontamination process. In the oil recovery, clays can be separated from the emulsion by simple centrifugation or sedimentation process being of larger density compared to water and micron-sized particles, of course this is not the case for conventional surfactants that can be hardly separated from the emulsion. As concerns metal adsorption, the ion release can be achieved aer purication by controlling pH and/or ionic strength that typically affect the metal-clay interactions.
Thus, the achievements of nanotechnology can be used for bioremediation, directly or indirectly, for the treatment of surface water, groundwater and wastewater polluted with toxic metal ions, organic and inorganic solutes and microorganisms. Due to their unique activity, nano-sized particles increase the efficiency of absorption of pollutants and are relatively inexpensive compared to traditional sedimentation and ltration methods.

Comparison between halloysite and kaolinite
Clay nanoparticles represent suitable adsorbent materials for the water ltration as shown in several research articles 46,47 and reviews. 48 In this regards, halloysite was successfully used for efficient water ltration as demonstrated in adsorption studies using cationic Rhodamine 6G and anionic Chrome azurol S as cationic and anionic dyes, respectively. 46 Fig. 1a sketches the ltration tests. Halloysite exhibited a higher removal capacity towards both dyes (Fig. 1b) due to its larger specic surface area (40.2 m 2 g À1 ) if compared to that of kaolinite (21.3 m 2 g À1 ). Both nanoclay based lters can be regenerated up to ve times by burning the adsorbed dyes.

Water ltration by electrospun membranes based on polyacrylonitrile and halloysite
The addition of halloysite nanotubes into polyacrylonitrile (PAN) revealed as an efficient strategy to obtain nanobrous membranes with excellent water ltration capacity. 47 As displayed in Fig. 2, the nanotubes are well dispersed within the PAN brous matrix.
A signicant enhancement of the oil removal efficiency was induced by the addition of halloysite into the PAN membrane. The removal efficiency of the membrane based on pure PAN was 4.7%, while the PAN/halloysite (99 : 1) and PAN/halloysite (97 : 3) composites showed adsorption capacities of 10.6 and 31.1%, respectively. These results highlight that the addition of small amounts of nanotubes into the polymer induce signicant improvement in the water ltration capacity of PAN based membrane. Moreover, PAN reinforced with 3 wt% of halloysite showed an increase by 740% for the heavy metal adsorption. Besides the enhanced adsorption capacity, the lling with halloysite caused relevant improvements of the thermal stability and mechanical properties (in terms of elongation and tensile strength) of PAN membranes.

Alginate/pectin gel beads for removal of heavy metals
The combination of negatively charged biopolymers (alginate and pectate) was explored to fabricate hybrid gel beads to obtain novel heavy metals adsorbents. 49 The composite gel beads were prepared through the dropping technique. Mixed gel beads exhibited lower density if compared with that of alginate based gel beads. On the other hand, gel beads containing pectate demonstrated the improved mechanical resistance compared to that of pure alginate beads as a consequence of the more compact structure. Adsorption tests demonstrated that the alginate/pectate system in gel phase can be considered a suitable and environmentally safe material for the removal of cadmium(II) and copper(II) from aqueous solution. Fig. 3 shows SEM images of alginate/pectate (1 : 2) beads before and aer the sorption of both cadmium(II) and copper(II) ions. As a general result, the surface of the beads was not signicantly altered by the adsorption of the metal ions.

Alginate/halloysite gel beads for water decolouration
Composite gel beads based on alginate and halloysite nanotubes were prepared by the dropping technique. 50 As shown in Fig. 4a, the gel beads size was not signicantly affected by the halloysite fraction, while their transparency is being reduced upon the addition of the nanotubes.
As shown in SEM images (Fig. 4b), dried hybrid beads possesses a rough surface with pores in the micrometre range that is similar to that observed in the absence of HNTs. 49 Water decolouration performances of the alginate/halloysite beads were tested by using Crystal Violet (CV) as dye. The adsorption isotherm of CV onto alginate gel beads shows an interesting dye extraction ability from the aqueous phase. Table 1 shows that the presence of halloysite in the gel beads improves the adsorption capacity (expressed in terms of maximum adsorption capacity (q max )). Regarding the adsorption constant (K), it was detected that the dye affinity towards hybrid beads is greater than that towards the pure alginate.
Gel beads based on alginate and halloysite were effective in the removal of methylene blue from aqueous phase.  Specically, the removal efficiency was above 90%. 51 Similarly to alginate/halloysite systems, the combination of clay nanotubes with chitosan allowed to obtain composite hydrogel with excellent adsorption capacity towards different dyes (such as methylene blue and malachite green) solubilized in water. 52 Chitosan/halloysite gel beads were prepared by using the dropping and pH-precipitation technique. The addition of halloysite signicantly improved the adsorption capacities (72.60 and 276.9 mg g À1 for methylene blue and malachite green, respectively) of chitosan based gel beads. 53

Kaolinite/graphene composites for remediation
In a recent paper, planar kaolin nanoclay was demonstrated to signicantly reduce the toxicity of graphene oxide nanoplates in the aqueous phase. The authors conducted studies on the ciliate Paramecium caudatum to identify the effect of different concentrations of the planar kaolin nanoclay and graphene oxide nanoplates. The nanoparticles used by the authors had a similar size distribution (hydrodynamic diameters are approximately 1.9-2.2 mm), and also both had negative zeta potentials (À47 mV for graphene oxide and À22 mV for kaolin). The introduction effect of graphene and graphene oxide into the environment, separately and in combination, was evaluated on the following physiological parameters of ciliates: chemotaxis, galvanotaxis, growth rate, DNA complexation, phagocytic activity. The authors emphasize that the toxicity of graphene oxide nanoplates which coagulated with kaolin was reduced without the removal of nano-conglomerates from the environment. At the same time, graphene oxide plates were highly toxic for P. caudatum in the concentration range from 500.0 to 1000.0 mg ml À1 . The GO concentration at 1000.0 mg ml À1 reduced the survival rate of ciliates and 55% of them died aer 24 h. At the same time, the addition of graphene oxide plates to the incubation environment with planar kaolin nanoclay and incubation for 24 hours signicantly reduced the negative effect of graphene oxide. For example, in the presence of kaolin with adsorbed graphene oxide at a concentration of 1000.0 mg ml À1 , only 7% of the protists died aer 24 h. That is, aer the adsorption of graphene oxide by kaolin, the toxicity of graphene oxide in an aqueous environment decreases by approximately 7.8 times.
Among the methods used to study the distribution of graphene oxide and kaolin in P. caudatum cells, dark eld microscopy with hyperspectral mapping was applied. 54 Spectral libraries of kaolin and graphene oxide were collected, which were subsequently used to analyse the distribution of both types of nanoparticles in living cells of P. caudatum (Fig. 5). Graphene oxide nanoplates and planar kaolin nanoclay nanoparticles were diffusely distributed in the cytoplasm, in the digestive vacuoles and in the macronucleus. As a result, the authors showed that kaolin signicantly reduces the toxic effect of graphene oxide associated with the membrane. It is assumed that planar kaolin nanoclay interacts with graphene oxide plates; it leads to a weakening of the chemical properties of the latter. The mechanisms of nanotoxicity of graphene-related materials have not yet been fully studied. Since both types of nanomaterials have negative zeta potentials, the electrostatic interaction between the studying particles is unlikely to be effective, as was previously demonstrated for heteroaggregation of graphene oxide with montmorillonite, kaolinite and goethite. 55 However, atomic force microscopy was successfully used to demonstrate that planar kaolin nanoparticles strongly aggregate with graphene oxide plates. 56 Thus, for the rst time, the authors of this work 56 describe a signicant reduction in the toxicity of graphene oxide plates using planar kaolin nanoclay in an aqueous medium. Using atomic force microscopy, it has been shown that kaolin coagulates with graphene oxide in water, forming relatively large conglomerates, which reduces the side negative effects of graphene oxide on P. caudatum ciliates, which potentially has high practical signicance in the eld of graphene materials.

Nanosponges based on halloysite and cucurbituril
Nanosponges composed of halloysite and cucurbituril molecules were prepared in order to fabricate biocompatible materials with excellent adsorption capacity towards hydrocarbons. Specically, cucurbit [8]uril (CB [8]) 57 and cucurbit [6]uril 58 were employed for the functionalization of halloysite nanotubes. As sketched in Fig. 6, the supramolecular complex between halloysite and CB molecule was prepared by mixing halloysite with a saturated solution of CB in water, which has low viscosity. The CB/HNT dispersions was stirred and kept under vacuum for 3-5 min. Then the vacuum was broken, solution entered into lumen and loaded compound condensates within the tube. This was repeated 2-3 times to increase the loading efficiency. Aer loading, tubes were washed several time with water in order to remove the CB did not interact and dried under vacuum at 70 C (Fig. 6).
The procedure described allowed generating modied HNT with a relevant functionalization degree. Based on thermogravimetric analysis, it was estimated that the amounts of CB [8] and CB [6] adsorbed onto the halloysite surfaces were 25 and 39 wt%, respectively. As shown in Fig. 6, the adsorption of CB molecules did not affect the peculiar tubular morphology of halloysite.
The supramolecular structures were tested for entrapping toluene both in the vapour and liquid phase. As concerns the adsorption experiments in vapour phase (Table 2), the hybrid nanomaterial exhibited a strong enhancement of capability in capturing toluene gas compared to the pristine HNT. At a given time, the adsorbed amount is larger for the nanohybrids; for instance, aer 2 h, the toluene amount captured by the nanohybrid is ca. 50 times larger than that of pristine HNT.
Adsorption experiments in liquid phase evidenced that the presence of CB [8] onto the nanoclay surfaces signicantly improves the HNT removal ability towards toluene dissolved in water. Spectroscopy measurements provided the partition coefficient of toluene (P) in aqueous dispersions of pristine and modied HNT. It was detected that P for CB [8]/HNT is three times larger with respect of that calculated for pristine HNT. Particularly, P values are 215 AE 8 and 72 AE 5 for HNT/CB [8] and HNT, respectively. These results indicate that the modied nanotubes possess a larger affinity toward toluene because of the presence of hydrophobic domains (composed by CB [8] molecules) on HNT surfaces. On the other hand, the lower adsorption ability of the pristine nanotubes correlates well with the HNT hydrophilic nature. The mechanism of interactions between HNT/CB [8] and hydrocarbons was clearly conrmed by uorescence spectroscopy experiments, which were carried out by using pyrene as a uorescent probe.    (sodium dodecanoate NaC12, sodium tetradecanoate NaC14, sodium dodecylsulphate NaDS and sodium per-uoroalkanoates). The functionalized HNTs can be considered as removal agents for decontamination purposes due to their capacity to entrap aliphatic and aromatic hydrocarbons inside their hydrophobically modied lumen. 38 Both the head polar group and the length of the hydrocarbon chain affect the surfactant loading. According to the HNTs sizes, the maximum loading value expected from the cavity is ca. 10 vol%, 63 which is almost reached for NaC12/HNTs and NaC14/HNTs hybrids. Namely, loading of the HNTs lumen with the formation of surfactant complex structures was detected for sodium alkanoates with longer alkyl chain. The lower amounts of NaDS and sodium peuoroalkanoates entrapped into the HNTs cavity are in agreement with a surfactant monolayer adsorption by taking into account the average specic area of the halloysite inner surface (6.9 m 2 g À1 ) 38 and the occupied area of the carboxylate and sulphate head groups. SANS data (Fig. 7) conrmed that the structural organization of the surfactants adsorbed onto the HNTs lumen depends on their polar head group. 60 In particular, SANS curves of NaC12/HNTs showed a peak at q (magnitude of the scattering vector) ¼ 1.79 nm À1 , which could be correlated to the formation of multilayers structures or cylindrical packing of surfactant within the HNT lumen. The formation of complex surfactant structures within the halloysite lumen improved the solubilisation ability of halloysite towards hydrocarbons. 38 As for the pure halloysite, nanotubes modied with NaC12 and NaC14/exhibited an enhancement of the toluene removal efficiency of ca. 9 and 18%, respectively. Moreover, the surfactant/halloysite composites were efficient in the removal of liquid n-decane as a consequence of the hydrophobization of the lumen. 38 Thermogravimetric volatilization experiments on n-decane equilibrated with halloysite powders highlighted the connement of the aliphatic hydrocarbon within the lumen of NaC12/HNTs. The capability to remove an oil lm at the water/air interface by using surfactant/HNTs systems was proved by time-resolved surface tension measurements (Fig. 8). This property was not observed for pure halloysite because of its hydrophilic nature.

Pickering emulsions for oil spill remediation
The formation of a Pickering emulsion (oil-in-water) stabilized by nano-or/and micro-particles can be considered as an alternative approach to increasing the surface area of the oil. This is relevant, for example, in terms of incident happened in the Gulf of Mexico. 64 Among the particles that can form the boundary between oil and water are described silica, latex, clay, even bacterial cells 65,66 and halloysite clay nanotubes are also suitable candidates for such Pickering emulsions formation. The formation of emulsion with crude oil was shown previously. [67][68][69] Native halloysite nanotubes have an electrical zeta potential of about À30 mV and a contact angle of about (13 AE 2) , but it is possible to modify the surface properties of the halloysite mineral nanoclay by its silanization and increasing the contact angle to (99 AE 30) . Hydrophobization of the inner surface of HNTs with silanes increases the stability of the emulsion. 69 It has been shown that the loading of widely used surfactants (Span 80, lecithin, etc.) into the lumen of nanotubes of halloysite signicantly increases the dispersibility of crude oil. 70  Alkanes constitute the largest part of crude oil by weight, that is why the biodegradation of this fraction is the most crucial when removing crude oil from the environment. In this study, the authors initially describe the use of native and silane-modied halloysite nanotubes for the emulsication of the nhexadecane model alkane and demonstrate that this approach works on Macondo crude oil. Moreover, the authors show that the use of the natural mineral halloysite attracts alkanedecomposing bacteria A. borkumensis and stimulates the viability of A. borkumensis cultivated with hexadecane or crude oil acting as the sole source of carbon.
The authors estimated bacteria viability by evaluating the growth of A. borkumensis in marine broth supplemented with nhexadecane (or crude oil) as the sole source carbon in the presence of the native mineral halloysite and ODTMS modied halloysite (HNT 99 ). Using the growth curves, the growth rate and the doubling time of the microorganisms were calculated. The doubling time (generation) is the time required for the cells to double their numbers. The average growth rate of bacteria in the control sample was 0.3 per hour, which corresponds to a generation time of 2.3 hours, while the growth rate of bacteria in an environment with nanomaterials of 0.25, 0.5 and 1 wt% was 0.28 hours, which corresponds to a generation time of 2.5 hours. These data are consistent with the growth rates of A. borkumensis on alkanes of different lengths (the growth rate of bacterial n-hexadecane is 0.3 hours). 71 Thus, the addition of halloysite aluminosilicate slightly increases the generation time. This is probably due to the fact that bacteria require more time for transition into the medium containing colloidal particles. We did not monitor signicant difference in growth rate and generation time in cultures supplemented with hydrophobised halloysite ODTMS (contact angle 99 ). Then, the effect of the halloysite mineral on the metabolic activity of A. borkumensis was investigated. The metabolic activity of bacteria plays an important role in assessing the ability of cells to consume and convert nutrients. This is not only an indicator of cell viability, but also an activity that is relevant for the survival of bacterial enzymes, such as dehydrogenase.
To assess the metabolic activity of bacteria in the presence of halloysite nanotubes the authors used in vitro method for assessing the activity of dehydrogenase enzymes that reduced non-uorescent blue resazurin to uorescent pink resorun. 72 It was made to demonstrate the kinetics of the production of resorun by bacteria in the presence of 0.2 mg ml À1 halloysite for 40 hours. The growth of A. borkumensis in both cases was supplemented with 1% hexadecane. The analyte was ltered before analysis to eliminate interference in resorun. The supplement of native mineral halloysite signicantly increased the rate of resorun formation, which indicates an increase in the activity of bacterial enzymes. Efficiency of metabolism causes a spurt of enzyme activity aer 24 hours of incubation with halloysite, when the rate of production of resorun is 2-4 times higher than the proliferation of bacteria at the oil/water interface. We can see that the metabolism of A. borkumensis increases in the medium with the supplement of halloysite. A. borkumensis is a common marine bacterium that degrades the aliphatic portion of crude oil. Therefore, we used n-hexadecane as the primary model of an aliphatic source of carbon. Aer 2-3 days of growth in a liquid culture medium the formation of white akes, oating in the upper part of the culture medium, is observed. A microscopic observation of these akes revealed a biolm containing a microemulsion of an oil droplet with a diameter of 10-50 mm. Examination of the sample under an optical microscope with an immersion objective showed that oil droplets contain short (1.5-2.5 mm) rod-shaped bacterial cells.
Then, the authors formed n-hexadecane emulsions in the marine broth containing ODTMS-hydrophobised halloysite nanotubes (contact angle of 99 ), aer inoculation with A. borkumensis to a nal concentration of 10 7 cells per ml. Aer 5-7 days the emulsions containing bacteria were observed. It is seen that the bacteria lengthen and form a dense patterned biolm on the surface of the oil droplets. Biolm is formed as a result of the growth of bacteria between halloysite aggregates. To test the ability of A. borkumensis to proliferate on the surface of crude oil emulsions as compared with detergent-based surfaces, oil emulsions with ablation of articial organic dispersants, as well as halloysite, were stabilized. The mixture of dispersants consisted of 48% non-ionic and 35% anionic substances similar in composition to one used during the oil spill in Horizon. 73 Subsequently, the mixture was blended with similar compounds, such as corexit, which were used during the spill.
Microscopic studies of these emulsions for one and three days aer the supplement of bacteria showed that several bacterial cells were attached to the oil/water interface in emulsions obtained with the dispersing mixture and (Fig. 9) one to three days aer inoculation. In samples containing native halloysite, attaching of cells to emulsions occurred at the same time (Fig. 9). We did not observe further cell proliferation at the interface in the presence of a dispersing mixture (Fig. 9), while the bacteria formed a dense biolm in the sample containing hydrophobized halloysite aer inoculation. The density estimate of A. borkumensis with a diameter of 20 microns increased ve times from 0.3 to 1.5 bacteria per m 2 (AE20%, as estimated by direct calculation).
The density of A. borkumensis did not increase so dramatically within oil emulsions based on native halloysite. However, we observed the formation of microbubbles with a diameter of 2-3 mm on the surface of large droplets of oil (Fig. 9). The absence of bacterial cells at the interface in the case of a mixture of surfactants may be stipulated by the repulsion of negatively charged bacterial cells using anionic surfactant DOSS or lysis of bacterial cells under the action of surfactants. It was found that DOSS (also known as AOT) at a concentration of 1.8 mm (0.08 wt%) suppresses the growth of A. borkumensis. 74 We used 1 wt% dispersing mixture containing 35 wt% DOSS and nonionic surfactants, which resulted in a nal DOSS concentration of 0.35 wt%. Similar growth was observed in samples of pure oil without halloysite. However, the bacterial mixture had to be well mixed in order to get a drop of emulsion small enough to observe the bacterial growth. In the case of an oil spill, dispersion of droplets with a diameter of 2 mm to 20 mm through halloysite Pickering stimulates a million times more surface area littered with hydrophobized halloysite, which contributes to bacterial prolongation emulsication.

Bioremediation technologies
Heavy metals are among the most dangerous pollutants and toxic elements for humans and the environment; among them, lead (Pb) and cadmium (Cd) are identied as the most negative ones. It has been noted that lead and cadmium are not decomposable and tend to accumulate in the food chain; therefore, for many countries, the issue of removing these elements from the environment is of paramount importance. Lactic acid bacteria (LAB) and lactobacilli in particular, have the ability to bind heavy metals, which makes them a promising tool for cleaning the environment and food industry products from heavy metals. Thus, based on the physicochemical properties of heavy metal elements, it is possible to specically bind them with microorganisms, which proves to be an environmentally safe, inexpensive and effective method of lead and cadmium removal. 75 A number of studies have shown that the accumulation of metal ions occurs on the cell surface due to physical adsorption, 76 but there is no data on the accumulation of heavy metals in the cell during such adsorption.
The authors of this study have characterized the surface of microorganisms according to their potential ability to extract cadmium and lead from different liquids. In the research work, ten Lactobacillus strains, including four L. plantarum strains, three strains of L. fermentum, L. brevis, L. buchneri and L. rhamnosus have been investigated. Some of these strains were isolated from probiotics, dairy products and silage. Hydrophobic/hydrophilic surface properties of microbial cells, solvent adhesion and electrostatic properties of cell surface have been identied. In this work, the lowest investigated concentration of cadmium was 5 mg l À1 and it did not affect the growth of lactobacilli, with the exception of L. fermentum 3-2. This strain has shown decreased optical density values. The increase of cadmium concentration to 10 mg l À1 led to a decrease in optical density values for L. plantarum 8PA3, L. plantarum j-578, and L. fermentum 3-2, occurring in its stationary phase, as well as to complete growth inhibition in L. brevis 20054, L. buchneri 20057 and L. rhamnosus I2L. The highest concentration of Cd used in the experiments was 50 mg l À1 . This concentration was toxic for all the studied samples. In the cultures of L. plantarum and L. fermentum, a signicant reduction of growth has been revealed, whereas in the cultures of L. brevis 20054, L. buchneri 20057 and L. rhamnosus I2L, growth was inhibited for 18 hours. The addition of Pb in 5 mg l À1 and 10 mg l À1 concentrations for all the studied strains caused the inhibition of growth in microorganisms as compared to the control strain. However, the addition of lead in the above concentrations did not cause changes in the optical density values of Lactobacilli, except L. plantarum S1 (where the addition of lead has entailed an OD600 decrease by 14.5% and 30.4% for 5 mg l À1 and 10 mg l À1 of Pb, respectively). The highest test concentration for lead was also 50 mg l À1 . The addition of this Pb concentration resulted in 100% growth  The authors also measured the zeta potentials of microbial cells' surfaces. Surface charge in the studied strains was negative: the lowest for L. plantarum S1 was À34.9 AE 6.8 mV, and the highest charge was recorded for the strain of L. fermentum 3-3, which amounted to À7.4 AE 0.9 mV. All the measured values are presented in the table (Table 3). Importantly, the zeta potentials of the microbial cells' surfaces differed signicantly between species and strains. Aer 1 hour of incubation in aqueous solutions containing 10 mg ml À1 of Cd or Pb, all strains of Lactobacillus showed a decrease in negative values of the cell surface zeta potential aer incubation with Cd and Pb, but these changes were not statistically signicant.
The authors have also demonstrated that the studied Lactobacillus strains were resistant to Cd and Pb. The following strains proved to be the most susceptible to heavy metals: L. brevis 20054, L. buchneri 20057 and L. rhamnosus I2L, because they showed a signicant decrease in growth when cadmium was added at a concentration from 10 mg l À1 to 50 mg l À1 , whereas L. fermentum and L. plantarum strains continued to grow at these concentrations. Most microorganisms studied were relatively hydrophilic (Table 3), which is consistent with the reference sources data. 77 Three strains of L. plantarum B-578 (52.0 AE 6.4%), L. brevis 20054 (63.1 AE 5.6%) and L. buchneri 20057 (66.9 AE 6.3%) were hydrophobic. The results of this study expand our the knowledge about the cell surface of lactobacilli and reveals the potential for the decontamination of Cd in four L. plantarum strains and three L. fermentum strains.

Conclusions
Clay nanoparticles are largely employed as adsorbent nanomaterials for pollutants because of their morphological characteristics, tuneable surface chemistry and sustainability. Their combination with organic molecules opens a new route in the fabrication of removal agents with targeted capacities towards aliphatic/aromatic hydrocarbons, organic dyes and heavy metals. Within this, hybrid gel beads formed by biopolymers (such as alginate and pectin) and halloysite nanotubes revealed as efficient systems for water decolouration, while the combination of kaolinite nanosheets and graphene oxides exhibited excellent remediation capacities. Supramolecular nanomaterials based on halloysite nanotubes and cucuriburil was successful in the toluene adsorption from both vapour and aqueous phase. The modication of halloysite cavity with anionic surfactants generated inorganic tubular micelles with a conned hydrophobic site efficient in the adsorption of aliphatic hydrocarbons including n-decane. Pickering emulsions based on halloysite nanotubes were tested for oil spill remediation, while novel bioremediation technologies were developed using lactic acid bacteria and Lactobacilli. Finally, clay nanomaterials are promising with respect to their reusability as they can be easily separated from water by simple sedimentation and adsorption/desorption phenomena can be reversed by controlling external parameters such as pH and ionic strength.

Conflicts of interest
There are no conicts to declare.