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Interactions of Dissolved Organic Matter with Natural and Engineered Inorganic Colloids: A Review Allan Philippe and Gabriele E. Schaumann* Institute for Environmental Sciences, Group of Environmental and Soil Chemistry, University Koblenz-Landau, Fortstraße 7, D-76829, Landau, Germany

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ABSTRACT: This contribution critically reviews the state of knowledge on interactions of natural colloids and engineered nanoparticles with natural dissolved organic materials (DOM). These interactions determine the behavior and impact of colloids in natural system. Humic substances, polysaccharides, and proteins present in natural waters adsorb onto the surface of most colloids. We outline major adsorption mechanisms and structures of adsorption layers reported in the literature and discuss their generality on the basis of particle type, DOM type, and media composition. Advanced characterization methods of both DOM and colloids are needed to address insufficiently understood aspects as DOM fractionation upon adsorption, adsorption reversibility, and effect of capping agent. Precise knowledge on adsorption layer helps in predicting the colloidal stability of the sorbent. While humic substances tend to decrease aggregation and deposition through electrostatic and steric effects, bridging-flocculation can occur in the presence of multivalent cations. In the presence of DOM, aggregation may become reversible and aggregate structure dynamic. Nonetheless, the role of shear forces is still poorly understood. If traditional approaches based on the DLVO-theory can be useful in specific cases, quantitative aggregation models taking into account DOM dynamics, bridging, and disaggregation are needed for a comprehensive modeling of colloids stability in natural media.

1. INTRODUCTION Natural inorganic colloids are present in almost all surface waters.1,2 Due to their high mobility and surface area, they play a crucial role in element cycling,1,2 pollutant transport,2,3 and they interact with the microbial communities.4−11 Therefore, they are part of the ecosystem and precise knowledge of their fate is crucial for the global understanding of aquatic systems. Natural colloids are mainly composed of iron oxides, aluminosilicates, quartz, aluminum oxide, and manganese oxide.2 Furthermore, the recent commercialization of nanoparticle-containing products has raised concerns about their possible accumulation and fate into the environment.12−14 For instance, typical engineered nanoparticles which could reach surface waters are TiO2,15 Ag(0),16,17 and Fe(0).18,19 Since the processes occurring in natural water are common to both natural colloids and engineered nanoparticles, we merge these two groups. However, we refer to differences in material and coating composition, which are crucial for understanding the distinct behavior of natural and artificial colloids or nanoparticles. Dissolved organic materials (DOM) are present in almost all aquatic ecosystems at concentrations typically ranging from 0.1 to 10 mg L−1 and depending on biogeochemical conditions and climate.20,21 Three categories represent together the most important part of DOM in surface waters: humic substances, polysaccharides, and proteins21,22 with highly different © 2014 American Chemical Society

molecular properties (Table 1). Humic and fulvic acids can be regarded as supramolecular assemblies of several thousands of different molecules.21,23 The high diversity and complexity of these assemblies constitute a challenge for the DOM characterization. DOM have a crucial role for colloid fate by modifying their surface properties and consequently their stability.27 DOM influence the transport in soils of various colloids such as hematite,28 clays,29 Fe(0),30 Cu(0),31 TiO2,32 ZnO,33 and hydroxyapathite.34 In addition, DOM strongly influence the adsorption of U(VI),35 Th(IV),36 Eu(III),37 Cd(II),38 Hg(II),39 and polycyclic hydrocarbons40−43 on colloids. DOM are also crucial for the removal of colloids using flocculation,44−46 biosorption,47−49 or filtration50−52 throughout water treatment. A clear understanding of the DOM−colloid interactions is therefore crucial to fully elucidate fate and transport mechanisms of colloids and pollutants in the environment. Although there is an abundant literature about these interactions, an exhaustive review of the relevant mechanisms lacked and rendered it difficult to draw general conclusions and identify knowledge gaps. This review points out such gaps and Received: Revised: Accepted: Published: 8946

May 12, 2014 July 9, 2014 July 31, 2014 July 31, 2014 dx.doi.org/10.1021/es502342r | Environ. Sci. Technol. 2014, 48, 8946−8962

Environmental Science & Technology

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Table 1. Indicative Molecular Parameters of Different Types of Natural DOM.20,23−26 DOM-type

molecular weight [kDa]

most common functional groups

charge (4 < pH < 10)

solubility in water

humic acids fulvic acids carbohydrates proteins fatty acids amino acids

2−5 0.5−2 0.18−3000 10−a few 1000 0.25−0.85 8).84 If Ca2+ was present, the adsorbed layer tended to contract because of intramolecular cation-bridging.42 Multilayer adsorption due to formation of hemimicelles has been proposed by Amal et al. to explain their adsorption isotherms of a fulvic acid.86 Limited desorption of total DOM and humic acids has been reported and a hysteretic behavior was observed.81,82,87 This behavior has been explained by the preferential adsorption of larger molecules than smaller.87 Some efforts were made to model the adsorption of fulvic acids on hematite88 but only electrostatic and van der Waals forces were taken into account in this model. 3.6. Goethite. The PZC of goethite is generally measured between 7 and 8.60,89,90 Therefore, hematite and goethite were observed to have a similar behavior for the DOM sorption. Electrophoretic mobility and AFM investigation suggested that DOM formed an approximately 5 nm thick coating on goethite leading to charge reversal of the surface.89,91 Ligand exchange and H-bonding with carboxyl groups were observed to be the major contributors to the adsorption of humic substances throughout sorption experiments.90,92 Electron paramagnetic resonance and isothermal calorimetry gave direct evidence that ligand exchange occurs between carboxyl groups from humic acids and hydrous ferric oxide.35 Combining experiments and models, Filius et al. gave strong arguments that H-bonding is the most important sorption mechanism for humic substances onto goethite followed by ligand exchange and electrostatic interactions. 93,94 Their model predicted accurately the adsorption pH and ionic dependencies and determined the proportion of inner and outer sphere complexes formed by carboxyl and hydroxyl groups of fulvic acids.93,94 Bridging with divalent cations was observed to increase the adsorption onto goethite for pH > 4. Sulfate and silicate had few effects,90,95 whereas phosphate ion was thought to compete

with DOM90 and especially with fulvic acids,96 although this observation contradicts results from another study.97 Desorption by increasing pH was observed for fulvic acids.98 The fractionation of DOM upon adsorption onto goethite has been discussed intensively with contradicting conclusions. While Tipping did not observe any difference in the adsorption pattern of three different DOM,99 experiments with fractionated DOM95,98,100 and molecular modeling101,102 gave strong evidence that the adsorption of humic substances increased with increasing molecular weight, aromaticity, and charge density. Sorption kinetics of fulvic acids fractionated by size demonstrated that high molecular weight molecules are not readily adsorbed onto goethite but replace lower molecular weight molecules already adsorbed over time.100 Sampling time is therefore an important parameter for the monitoring of DOM fractionation. This latter point may explain that opposite results using various DOM can be obtained as sampling time differs from one study to the other.103 3.7. Magnetite. Although magnetite (Fe3O4) has an PZC around 8104,105 like hematite, different adsorption isotherm patterns and thus different mechanisms for DOM were observed for magnetite and hematite.105,106 A multilayer adsorption due to hemimicelles formation may explain these differences in accordance with the observation than carboxyl and aliphatic moieties participate to the adsorption of humic acids.104,107 Similarly to hematite, ligand exchange and electrostatic interactions could reliably explain the pH and ionic strength dependencies of the adsorption isotherms of humic acids.104 In contrary to hematite, preferential adsorption of smaller molecular weight molecules of humic acids has been reported for magnetite.104 The concentration of aromatic and carboxyl groups correlated positively with the amount of material adsorbed.104 It should be mentioned that, in most studies addressing magnetite, humic acids extracted from coal as sorbate were used.104−106 These types of humic acids may not fully reflect the behavior of natural DOM. Therefore, further investigations with different types of DOM are needed to completely understand DOM sorption onto magnetite and explain the differences with other iron oxides. 3.8. Aluminum Oxide. Aluminum oxides are often used as models for natural minerals. Their PZC depends on the phase composition and varies from 6 to 9.40,108−110 Ligand exchange with carboxyl and hydroxyl groups was demonstrated for various DOM and for α- and γ-Al2O3 using proton excess titration,111,112 IR-spectroscopy,110,113 and sorption isotherm at different pH.37,83,110,111 van der Waals and hydrophobic forces are also considered to contribute to the adsorption of humic acids83 and hydrophilic fractions of DOM.114,115 Indeed, adsorption isotherms and 13C NMR indicated the formation of hemimicelles on the sorbent surface.83,114,115 Interestingly, hemimicelles formation was also observed throughout the adsorption of bovine serum albumin onto alumina.116,117 The presence of divalent and trivalent cations was found to increase humic substances adsorption onto aluminum oxide.37,83 In contradiction, Davis observed a slightly weaker adsorption in the presence of Ca2+.111 Differences in the ionic strengths used in these studies and the absence of data on aggregation make the generalization of these findings uncertain. Competition for the sorption sites was observed for DOM with phosphate111 and carbonate ions.114 Concerning fractionation upon adsorption, Schlautman and Morgan observed that high amounts of humic acids adsorb compared to fulvic acids,83 in accordance with Davis who 8949

dx.doi.org/10.1021/es502342r | Environ. Sci. Technol. 2014, 48, 8946−8962


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demonstrated that DOM with higher molecular weight adsorbs better.108 Iorio et al. found a particle size dependency of the selectivity of polymerin adsorption.40 While microparticles (110 μm) promoted aromatic compounds, no preference was observed for nanoparticles (60 nm). These differences could be explained by the different geometries and levels of crystallinity of microparticles and nanoparticles40 or by thermodynamic considerations.118 These results demonstrate the importance of a thorough particle characterization prior to adsorption experiment. 3.9. Aluminum Hydroxide. The first adsorption study carried out with a fulvic acid on aluminum hydroxide revealed that carboxyl groups exchanged with hydroxyl groups (Al−OH) situated on the edge faces of the mineral but not on the (001) face, where the hydroxyl groups ((Al)2−OH+) are strongly bound to the surface.92 More recently Guan et al. confirmed that carboxyl groups contribute to attachment of fulvic and humic acids at low pH, while phenol groups become more important at high pH.119 The same authors found that high molecular weight molecules adsorb in higher amounts than smaller molecules but were more efficiently removed by phosphate anions,119 which is in contradiction with Borggaard et al.97 3.10. Titanium Dioxide. The PZC of TiO2 is generally found to be between 5 and 6.15,120,121 Bare TiO2 surfaces can hence be negatively or positively charged in environmental media. Increases in pH or ionic strength decreased and increased, respectively, adsorption of DOM.122 However, the adsorption of humic acids onto TiO2 was less affected by pH than aluminum and zinc oxides.110 Evidences from UV and IRspectroscopy indicate that phenol groups were responsible for the attachment of humic acids at the surface.110,110,122 However, carboxyl groups of smaller molecules can also form complexes with the surface of TiO2 nanoparticle.123 Erayhem et al. observed that Mg2+ and Ca2+ increased the adsorption of various DOM onto TiO2, probably through cation bridging and surface charge screening, while nitrate, carbonate and phosphate compete with DOM for the sorption sites.122 3.11. Silica. Since the surface of SiO2 is negatively charged at environmental pH,117,124 Coulomb forces are repulsive and the adsorption of DOM on SiO2 is generally weak.52,62,110,111,125 However, the high stability of quartz colloidal suspension toward aggregation124,126 and the decrease of sorption capacity for atrazine127 in the presence of fulvic acids both suggest that even a low amount of adsorbed fulvic acids can influence the physicochemical properties of SiO2 particles. In contrary to humic substances, high amount of proteins adsorbed on amorphous SiO2 particles with formation of multilayers for bovine serum albumin.117 The low intrinsic charge and the macromolecular character of BSA could explain this higher adsorption compared to strongly negatively charged humic substances. 3.12. Aluminosilicates. Aluminosilicates have been thoroughly studied because of their high relevance for soil and aquatic media. These minerals are often found as plates with positively charged edges and negatively charged faces at environmental pH.128 DOM adsorb in general less onto aluminosilicates than on goethite or aluminum oxide because of this unfavorable electrostatic pattern.95,129 Their PZC depends strongly on the mineral type. The PZC between 4.8 and 7 were found for kaolinite,95,109,130,131 while montmorillonite remains negative over the whole range of environmental pH.105

As observed for other metal oxides, adsorption increased with decrease of pH and increase in ionic strength for various NOM and minerals.41,42,129,130,132 Release of hydroxyl ions confirmed that ligand exchange took place at the surface of kaolinite, vermiculite, and smectite,42,133 at least for humic and fulvic acids, while opposite results were obtained with montmorillonite and compost leachate.134 Ligand exchange could be explained by chelation of aluminum ions situated on the edges by aromatic carboxyl acids and phenols for kaolinite and montmorillonite.109 Using X-ray photoelectron spectroscopy, Zhang et al. directly observed chelation of iron ions present on the surface of vermiculite by aromatic compounds. Similar binding with other ions could explain the preferential adsorption of aromatic compounds and oxygen containing compounds observed with humic substances for diverse aluminosilicates.41,95,129,133,135 Preferential adsorption was also observed for high molecular weight molecules of several DOM and for various minerals with one noteworthy exception found for compost leachate on montmorillonite.134 Increase in adsorption of various DOM on kaolinite and montmorillonite surfaces was observed in the presence of Ca2+.42,132,135,136 Majzik and Tombácz studied in detail this effect for humic acids and montmorillonite and explained it by cation bridging.135 These authors concluded that around 50% of the adsorbed humic acid was bound gradually in larger quantities through Ca 2+ bridging with increasing Ca 2+ concentration up to 5.5 mmol L−1 and they observed multilayer formation at high Ca2+ concentration related to the cation exchange capacity of montmorillonite.135 Limited desorption of humic substances has been reported.41,129,132 Interestingly, Zhou et al. observed that hydrophilic fractions of DOM (low aromaticity, high O/C content) desorb more than other fractions like fulvic acids and humic acids.129 In addition, the adsorption of fulvic acids was completely reversible, while humic acids did not desorb in their experiments.129 The compounds adsorbing in higher amount were also the most difficult to desorb.129 Thus, it seems that the adsorption of large, aromatic and aliphatic fractions of humic substances onto aluminosilicates is irreversible and that these fractions are hence thermodynamically favored compared to small hydrophilic molecules. 3.13. Other Colloids. ZnO begins to dissolve at pH < 8137 and its PZC was detected around 9.137 Its surface is therefore positively charged and saturated with zinc ions under environmental conditions. Therefore, humic substances readily adsorb onto ZnO. IR-spectroscopy experiments have indicated that humic acids bound to ZnO nanoparticles through aromatic and aliphatic hydroxyl and carboxyl groups.35,138 DOM from a compost leachate was reported to adsorb onto MnO2 through a ligand exchange mechanism by carboxyl groups with a high selectivity for aromatic compounds.134 No fractionation based on molecular weight was observed.139 DOM were partially oxidized on the surface of the solid explaining the high preference for electron donating aromatic compounds.139 Other manganese oxides (Mn3O4 and βMn(O)OH) were studied for their DOM sorption139 resulting in similar conclusions as for other metal oxides: increasing adsorption by decreasing pH and by the presence of Ca2+.139 For CdSe quantum dots and different HfxZr1−xO2 nanocrystals capped with hydrophobic coatings, both chelation and overcoating (accompanied by the formation humic acids micelles around the particle) were observed.140−142 In particular, the adsorption mechanism seems to depend strongly 8950



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on the crystal phase.142 To which extent current knowledge on micelle formation of HA23,143 can be used to understand selforganization of HA in the presence of inorganic colloids, is still unknown. 3.14. Conclusions about Adsorption and Research Needs. The mechanism of DOM adsorption on inorganic colloids highly depends on material and capping agent. Therefore, prediction of the nature and the properties of DOM coating is not possible on a general basis. However, it can be concluded that ligand exchange (mostly with carboxyl and phenol groups) and electrostatic interactions prevail beside other observed mechanisms for most metal oxides, with some noteworthy exceptions (e.g., magnetite). This is true at low pH. At neutral or high pH, cation bridging, especially by Ca2+, may play the major role under environmental conditions. Further works are needed for studying the effect of environmental divalent or trivalent cations on DOM adsorption as they may be highly efficient bridging agents, even at low concentrations. Surface charge and chemical affinity of the material contribute to determine the quantitative aspects of DOM-adsorption. The interplay between conformation and adsorption of DOM is complex and depends on all factors cited above. Indeed DOM can form a flat or extended layer on the surface depending on the surface charge, pH, ionic strength, and the presence of multivalent cations. Classical polyelectrolyte theory combined with metal-oxides chemistry seems to be a satisfying model for describing this behavior for most metal oxides. Nonetheless, further systematical studies are still needed for TiO2 and CeO2 which have a specific chemistry compared to most metal oxides. Since competition with several anions were studied and reported for a few number of systems only, further works should also consider the effect of environmental anions and especially sulfate, phosphate, carbonate, and nitrate. A more debated fact concerning metal oxides is that molecules or molecular assemblies with higher molecular weight adsorb in higher amounts than smaller ones, probably for entropic reasons. This fact linked to another almost general statement: the limited desorption of DOM suggests that a portion of the molecules adsorb irreversibly, although further displacement by specific anions cannot finally be ruled out. As shown in the first part of this review, there are only few studies about DOM adsorption onto metallic particles. Nonetheless, these few results suggest that the original particle coating can be in competition with DOM or form an intermediate between colloid surface and DOM. Although several studies addressed this question (with Ag(0), Au(0), Fe(0) particles, and quantum dots), quantitative information about removing or covering of coating is still lacking. However, the actual findings suggest that the nature of the capping agent may be more important than the particle composition for DOM adsorption. From a methodological point of view, there is a clear lack of characterization for some nanoparticles surfaces in several studies, especially Au(0), Ag(0), and Fe(0). Indeed, these surfaces can be impure, contain defects (in particular for nanoparticles) or being oxidized and hence the adsorption mechanisms can be completely different than with the bulk material. Moreover, the exact structure of coating is rarely reported. Knowledge about particle surface is thus crucial for determining the binding mechanism. Even more problematic is the lack of characterization of the suspensions used in most sorption studies. Indeed, the aggregation state, the average particle size, shape, and charge are often not determined, although they are crucial

for fully understanding the sorption mechanism. Despite these methodological questions, the results presented in this section are very useful for understanding the behavior of DOM coated colloids under different conditions, in particular the colloidal stability.

4. EFFECT OF DOM ON COLLOIDAL STABILITY 4.1. Colloidal stabilization and destabilization pathways. Understanding the fate of colloids in the environment requires quantifying their aggregation and deposition on surfaces.144,145 Aggregation occurs at short distances between two particles when the attractive van der Waals forces prevail on the Coulomb forces.59 Deposition is based on the same principle and applies to a particle and a surface. Surface modifications induced by DOM adsorption can shift the balance between these two forces and can create new types of interactions.146 A detailed description of aggregation and deposition theories with the influence of ionic strength, pH and colloid concentration has been reviewed elsewhere.145 Therefore, this section focuses on the effect of DOM on these phenomena. 4.2. Role of DOM for Colloidal Stabilization Mechanisms. 4.2.1. Aggregation in Environmental Media. Similarly to the adsorption of DOM onto colloids, the first studies addressing the effect of DOM on colloid aggregation were dedicated to natural colloids and pinpointed the ambiguous role of DOM. On one hand, DOM was observed to stabilize natural colloids in natural waters by increasing their electrostatic stability.147−150 On the other hand, bridging flocculation induced by long molecules can accelerate the aggregation of particles in surface water or in soil.149,151−153 An explanation to these antagonistic effects is that humic like molecules stabilize electrostatically the particles by coating their surfaces, while long polysaccharides and peptides induce flocculation.154,155 Similar observations concerning the stability of colloids were made in standard synthetic waters, natural waters, and biological test media. Thus, fulvic acids, 68,156 humic acids125,157−159 and total DOM121,160−163 have been reported to stabilize colloids in such media, while bacterial exudates (mainly polysaccharides and polypeptides) induced natural colloids sedimentation.164 These observations have to be relativized, since the effect of DOM on colloidal stability depends on various parameters as described in the next sections. 4.2.2. Electrosteric Stabilization. Most experimental setups involve monitoring the average size of suspended particles over time using dynamic light scattering.165 The stability ratio is a parameter describing the colloidal stability of a suspension and can be determined by monitoring aggregation or sedimentation over time. Comparing stability ratios measured under different conditions (including pH and electrolyte type) gives information on the important parameters dominating the aggregation.145 Due to the intrinsic negative charges present in most DOM, their adsorption onto colloidal surfaces imparts more negative charges to the surface. If the surface charge is initially positive, addition of small quantity of DOM can neutralize the positive charges and reduce the intensity of the Coulomb forces. Consequently, the adsorption of DOM can induce aggregation as long as stabilization is purely electrostatic. Destabilization was observed for particles which can be positively charged under environmental pH like hematite,86,124,166,167 magnetite,168 and TiO2.169−172 If the surface charge is initially negative 8951



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or if the amount of adsorbed DOM is enough for fully reversing the surface charge, colloids will be electrostatically stabilized. Aluminosilicates are special cases due to their heterogeneous surface charge distribution. DOM molecules adsorb preferentially onto the positively charged edges (see above) and efficiently inhibit the electrostatically favored edge-face aggregation.105,109,130,131,173,174 Steric hindrance due to large DOM molecules has to be added to this electrostatic effect. The stabilization due to the repulsion of two macromolecular layers is highly efficient if the adsorbed layer thickness is larger than the Debye length as particles cannot approach each other over the distance where van der Waals forces predominate.167 Adsorption layer thickness depends on the amount of adsorbed molecules at the surface as well as their conformation and is therefore influenced by media composition and DOM−surface interactions.41,42,167 If adsorbed DOM take a flat conformation on the colloid surface, the electrostatic effect tends to dominate, whereas steric effects will become more important if adsorbed molecules take extended conformations.167,175−177 Following the classical aggregation theory (see below), multivalent electrolytes are much more efficient than monovalent ions at compressing the electric double layer and, therefore, at inducing aggregation.59 Moreover, DOM can form complexes with many multivalent cations, which neutralize a part of their negative charges of DOM adsorbed on the surface and modify the conformation of DOM (coiling).41,42,135,178 Thus, multivalent cations can, in some cases, decrease the magnitude of electrostatic and steric stabilization simultaneously. A combination of electrostatic and steric effects (also called electrosteric effects) can qualitatively describe the stability behavior of most systems like, for instance, iron oxides,124,167,179 SiO2,124 Ag(0),180−182 Au(0),183 TiO2,169,171,172 ZnO,137 and various metal sulfides184−188 in the presence of humic substances or total DOM if additional mechanisms like molecular bridging or heteroaggregation do not participate. 4.2.3. Importance of DOM Composition for the Colloidal Stabilization. Concerning colloidal stabilization, it is interesting to consider the influence of compositional and structural differences of DOM stabilization. Comparison of various types of DOM for their stabilization effect led to similar conclusions although different materials were used. Large humic acids with lower oxygen content better stabilized hematite,189 Au(0),190 and MnO2191 particles than fulvic acids; probably because of the formation of a larger layer thickness inducing higher steric hindrances.190,192 The stabilization mechanism differs between the humic substances fractions. For instance, alumina particles were more stable in the presence of the less polar and aliphatic fractions of a humic acid than with polar fractions.192 Similar results were obtained with fulvic acids and Au(0) nanoparticles.190 Comparing diverse DOM for their stabilization, Huangfu et al. observed the following order in decreasing stabilization: bovine serum albumin, humic acids, fulvic acids, and alginate.191 Interestingly, these differences were not observed when Ca2+ was present indicating that, at higher ionic strength, complete destabilization may mask DOM differences.191 Among three different polysaccharides, guar gum stabilizes better Fe(0) nanoparticles, followed by starch and alginate.193 For the stability of iron based particles, fatty acids had less influence than humic substances,189 while rhamnolipids were better stabilizers than carboxymethyl cellulose and proteins.194

Beyond compositional differences, the configuration of the surface binding groups could influence colloidal stability.195 Unfortunately, this aspect remains unaddressed in most studies. Differences in molecular weight are also important as demonstrated by Stacey et al. with DOM fractionated by filtration.196 Indeed they have reported that the highest molecular weight molecules can dominate the stability behavior of Au(0) particles at high DOM/particles concentration ratio, although they represented less than 2% of the total DOM mass.196 Thus, it seems that large, aliphatic, and surfactant-like molecules are better suited for sterically stabilizing colloids upon aggregation. However, further studies are necessary for confirming this conclusion. 4.2.4. Molecular Bridging. Bridging is defined as a molecular connection between two particles and leads to morphologically specific aggregates. When such connections lead to particle aggregation, this phenomenon is called flocculation.59 The aggregation through cation bridging is usually faster than through pure electrostatic destabilization197−201 as the macromolecular coating enlarge the collision radius of the particles.202 The first reports of bridging mechanisms by DOM concerned natural colloids embedded in organic matrices.203−207 Electron microscopy, simulations, and experiments with model systems allowed the authors to conclude that large rigid biopolymers were responsible for the heteroaggregation of natural inorganic colloids in natural waters.203−205 Ferretti et al. observed this type of bridging with synthetic hematite particles and schizophyllan.208 Exhaustive characterization and aggregation simulation of their system demonstrated that particles connected the large rigid neutral chains of this polysaccharide together.208 Two DOM chains adsorbed on different particles can also connect through H-Bonding as observed for a polysaccharide rich humic acid fraction using atomic force microscopy.179 However, it is well-known that Ca2+ can form complexes with various humic substances.209,210 Connection through cation bridging, especially with Ca2+, is hence more common and was observed for fulvic acids,86 humic acids,178,179,197,199,200,211 polysaccharides,191,197,198,201,212,213 and soil-borne DOM.214 Ca2+ bridging could be directly observed with electron microscopy combined to electron diffraction analysis199 and AFM.212 Labille et al. studied the aggregation of clay minerals in the presence of structurally different polysaccharides and Ca2+.213 They have reported that the bridging effectiveness was influenced mostly by the amount and position of carboxyl groups in the polymeric chains.213 Polymers with carboxyl groups located on flexible side chains like succinoglycan were the most efficient floculants.213 In addition, cation bridging seems reserved to Ca2+ or heavier divalent cations (Sr2+, Ba2+);212 Mg2+ providing only electrostatic destabilization.198,201,212 However, other multivalent cations like Fe3+ or Al3+ have not been yet fully investigated for this aspect. Although Mg2+, Na+ and K+ do not form strong bridges between DOM molecules, they can compete with Ca2+ for the complexation sites of DOM and hence reduce their bridging efficiency.197,198 4.2.5. Modification of the Aggregate Structures Induced by DOM. DOM can influence the morphology of aggregates throughout their formation by modifying the particle surface charge and creating a steric barrier and thus modifying the aggregation regime. Fractal dimension analysis based on multiangle light scattering or electron microscopy is most 8952



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often used for quantifying the compactness of aggregates.215,216 Reaction limited aggregation (unfavorable conditions) produce compact spheroidal aggregates, while diffusion limited aggregation (favorable conditions) produce loose, fractal shapes.216 Hematite nanoparticles formed more compact aggregates in the presence of humic substances than without, while an opposite effect of DOM was observed with Ca2+ as electrolyte, with looser aggregates formed when humic substances was present.85,86,217 The increase in compactness has been explained by changes in the aggregation regime due to the steric stabilization by humic substances,85,217 while decrease in the presence of Ca2+ has been connected to the formation of loose flocs upon cation bridging.217,218 Indeed aggregates formed by cation flocculation are thought to be looser than diffusion limited aggregates.203 Chowdhury et al. have reported a complex dependency of the compactness of TiO2 aggregates formed by Ca2+ bridging on ionic strength, pH, and primary particle size in the presence of humic acids.219 Humic acids also prevented gold nanoparticles for fusing at low pH, preventing irreversible aggregation.220 DOM can also directly induce aggregates restructuration as demonstrated by Amal et al. for hematite nanoparticles.86 In this case the aggregates tend to become more compact over time.86 Although the modification of aggregates structure is highly important, few studies addressed this topic. One important reason for that is the difficulty to characterize such complex objects like aggregates. The development of dedicated analytical methods in addition to multiangle light scattering or artifact free electron microscopy (e.g., cryogenic transmission electron microscopy) would be highly helpful in this field. 4.2.6. Disaggregation. DOM adsorption onto aggregates can even induce disaggregation as observed for hematite and TiO2 nanoparticles.120,221 This phenomenon is well-known in soil science.222 By adsorbing onto the surface of aggregated particles, DOM can modify their surface charge and form a steric layer destabilizing the particle−particle connections and leading to disaggregation. Indeed, the importance for disaggregation of the sorbate thickness layer and, therefore, of its conformation was demonstrate for a polysaccharide by Yokoyama et al.177 Disaggregation kinetics depends on the capacity of the sorbate to diffuse into the aggregate structure and thus completely cover the particle surface. For instance, a long flexible polymer like alginate was observed to penetrate faster than a humic acid into the structure of TiO 2 aggregates.120 Diffusion limited aggregates disaggregate faster than reaction limited aggregates as, probably, only the weakest particle−particle connections can be broken.223 This can explain that disaggregation is generally not complete.120,221 Comparison of results obtained from two studies reporting disaggregation induced by polymers,177,224 suggest that orthokinetic factors like stirring intensity, can strongly influence the disaggregation process. Interestingly, all studies reporting disaggregation were carried out on stirred samples.120,177,221,223 Therefore, it is highly probable that shear forces are necessary for disaggregation to occur. However, a systematical investigation of the effect of shear forces on the stability of aggregates in the presence of DOM lacks up to now. 4.3. Role of DOM on Deposition of Colloids. Deposition (surface attachment) is conceptually related to heteroaggregation in that sense that the properties of two different surfaces have to be considered. This additional complexity may explain that deposition kinetics were less addressed than homoag-

gregation. However, recent methodological developments involving quartz crystal microbalance (QCM) and atomic force microscopy (AFM) have made quantitative exploration of these complex interactions easier.145,225 Most of reported studies on colloid deposition used quartz surfaces 209,226−230 but deposition onto alumina 231 and bacteria226,232 were also addressed. Deposition of hematite,209 ZnO,228 Ag(0),227 CeO2,229 and TiO2226,230 particles onto SiO2 considerably decreased in the presence of humic acids and similar conclusions were drawn for deposition of Fe(0)232 and TiO2226 onto bacteria and quantum dots onto alumina.231 Electrosteric repulsion and reduction of friction forces are most probably responsible for these decreases, as proposed by Phenrat et al., who developed an empirical model, based on extended DLVO-theory and Oshima’s soft particle theory, for the prediction of nanoparticles deposition in the presence of NOM.233 Humic acids were more efficient than fulvic acids in maintaining hematite209 and Ag(0)227 in solution due to the more efficient steric stabilization of the large humic acid molecules.209 Little differences between the abilities of humic acids, fulvic acids, polysaccharides, and rhamnolipids to stabilize CeO2 and quantum dots were observed.229,231 Once again the presence of Ca2+ plays a major role. It increases the deposition efficiency on SiO2 of hematite,209 Ag(0),227 ZnO,228 and TiO2226 in the presence of humic substances. Interestingly, the deposition of TiO2 nanoparticles onto bacteria in the presence of humic acids and exopolymeric compounds from the bacteria was favored over the deposition onto a SiO2 surface.226 Ca2+ bridging through exopolymeric compounds and humic acids probably occurred between bacteria and nanoparticles.226 This indicates that some nanoparticles may be preferentially adsorbed onto bacteria than onto mineral surfaces. 4.4. Application of DLVO theory to DOM Coated Colloids. The colloidal stability is traditionally described by the Deryaguin-Landau-Verwey-Overbeek theory (DLVO).59 The classical DLVO-theory considers van der Waals and Coulomb forces between the materials of the surfaces in contact and their respective electrical double layers for calculating the energy potential of the total interaction.59 In case of homoaggregation, the interaction energy between two approaching particles can be calculated and used for predicting diverse parameters including critical coagulation concentration, sticking coefficient and shear stress.59 These parameters can be determined by carrying out aggregation kinetics experiments.146 DLVO-theory is efficient for a wide range of colloidal suspensions in the absence of stabilizer.145 In order to predict additional nonelectrostatic interactions like steric hindrance, osmotic forces, and magnetic forces for instance, extended versions of the DLVO-theory were proposed.146,234 This section does not aim to describe extensively the concepts related to these theories but is thought as an overview of attempts to apply this theory to colloids in the presence of DOM. For an exhaustive mathematical and physical discussion of the DLVO-theory and its extensions we referred to the literature overviews written by Petosa et al.145 and Grasso et al.146 As the classical DLVO-theory does not include steric effects, it is hardly surprising that this model fails to predict the stability of colloids coated with macromolecular molecules. For instance, the stability of Ag(0) nanoparticles can be correctly predicted by this model if the particles are coated with citrate (electrostatic stabilization) but not if a polyvinylpyrrolidone 8953



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coating is present (steric stabilization).211 Similarly, these two types of Ag(0) nanoparticles do not follow the DLVO-theory in the presence of humic acids.211 This model also failed to describe the deposition of ZnO, CeO2, and TiO2 nanoparticles onto mineral surfaces in the presence of DOM.226,228,229 Strong limitations of this theory were observed for alumina particles by Harbour et al. as they attempted to describe the relation between shear stress and electrophoretic mobility in their experiments.175,176 Interestingly, these authors observed that discrepancies with the DLVO theory depended on concentration, fraction type, and origin of DOM.176 The non-DLVO behavior was more obvious at low DOM concentration and with hydrophilic and neutral fractions.176 These results can be explained by the different conformations that these different DOM molecules can take on the mineral surface as weak steric hindrance is expected for flat adsorbing molecules. In such cases the electrostatic aspects dominate and classical DLVO-approaches can succeed. In fact, humic acid-coated hematite particles follow the critical coagulation concentration dependency on the valence of electrolytes derived from the DLVO-theory (Schulze-Hardy rule), except for trivalent cations.235 Classical DLVO-theory extended by magnetic forces also succeeded in predicting the aggregation kinetics of magnetite particles.234 Concerning clay minerals in the presence of humic acids, Borgnino observed a good correspondence between DLVO-predictions and experimental results,173 in contradiction with Furukawa et al., who observed that the non-DLVO components of the interaction potential were higher that the electrostatic components.128,236 These discrepancies in those results obtained with clay minerals and humic acids are probably related to the different approximations used for building the DLVO-model.128,173 This remark illustrates the difficulty of using a simple model for describing complex particles (nonspherical shape, heterogeneous charge repartition) and complex macromolecules. Indeed several parameters like the Hamaker constant and the diffuse layer geometry are difficult to determine for such systems. Nevertheless, DLVO-theory extended to account for steric and osmotic interactions can be useful for modeling colloidal stability of well-characterized linear polymers like polysaccharides and polypeptides. For instance, Phenrat et al. managed to predict with a good confidence the stability of magnetic Fe(0) nanoparticles coated with such polymers having different molecular weights combining Oshima’s soft particle theory and extended DLVO-theory.237 Based on this method they proposed an empirical relation for estimating the attachment behavior of polymer coated nanoparticles in porous media.233 Although extended DLVO-theory can be accurately used in such cases, it cannot be used to predict disaggregation or bridging; two phenomena observed in experimental systems.211,221 For describing bridging and disaggregation, a molecular model for the adsorbed layer and its interactions (H-bonding, hydrophobic forces, electrostatic interactions, etc.) should be developed and combined with the extended DLVOtheory. Further development of Monte Carlo simulations based on coarse-grains model of the conformation of polyelectrolytes adsorbed on nanoparticles could be helpful for developing such models.238,239 4.5. Conclusions about the Effect of DOM on Colloidal Stability. Effects of DOM on colloidal stability were investigated for a large range of materials, particle types, and surfaces and some general statements are possible. These effects and their conditions are summarized in Figure 2 and Table 2.

Figure 2. Schematic description of the principal effects of DOM on the colloidal stability of particles reported in the literature.

Humic substances bring negative charges to the surface of particles upon adsorption. This can induce electrostatic destabilization if particles are initially positively charged; otherwise electrosteric stabilization will strongly hinder aggregation unless molecular or intermolecular bridging occurs and induces flocculation. These effects are qualitatively independent of the particle type. Cation bridging was regularly observed for various types of DOM and should always be considered in media containing multivalent cations. In the reviewed studies, deposition of different particles type onto surfaces was always strongly hindered by the presence of any DOM, although further work with different surfaces is required for drawing general conclusions. Although the most studies presented in this section address homoaggregation, heteroaggregation is highly relevant for environmental media as well.12,204,240 Therefore, investigation of the effect of DOM on heteroaggregation would be useful. Attempts to model complex systems with extended DLVO are promising but are still far from a comprehensive quantitative theory. Future models should take into account the cases of partial DOM coverage and the formation of molecular bridges.

5. ROLE OF DOM IN COLLOID AGING Dissolution is an important aging process for colloids in environmental media. Dissolution of various materials like Ag(0),241−244 iron oxides,245 ZnO138,246 or Cu247 have been reported. Dissolution can be influenced in several ways by DOM. Thermodynamic equilibrium can be shifted toward ions by chelation of the latter by DOM. One the other hand, the presence of a coating can constitute a significant kinetic barrier for the diffusion of ions away from the surface and can inhibit the diffusion of reagents, which are sometime necessary for the 8954



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Table 2. Listing of the Principal Effects of DOM on the Colloidal Stability of Particles with Their Respective Most Relevant Requirements for Particles, DOM, and Mediaa type of effect no adsorption electrostatic destabilization electrostatic stabilization electrosteric stabilization cation bridging rigid polymer bridging disaggregation a

particles Au(0) coated with polyvinylpyrrolidinone, SiO2 initially positively charged (iron oxides, Al2O3, TiO2...) any particle adsorbing DOM

DOM

media

no interaction with

classical DLVO

moderate ionic strength

DLVO extended with steric forces none

any particle adsorbing DOM

low DOM/colloid ratio, small or flat adsorbed molecule sufficiently high DOM/colloid ratio, small or flat adsorbed molecule high molecular weight, extended conformation, full surface coverage long flexible molecule with bridging sites


long molecule

presence of multivalent cations none

particles unable to adhere

flexible

shear forces, slow process


Model available

stability of the coating in the media pH < point of zero charge moderate ionic strength

classical DLVO classical DLVO

see refs 204, 208 none

None means that no knowledge is available for this parameter/model.

processes for evaluating the reactivity and the stability of metallic particles in the environment but are rarely addressed.72 Reactions like reduction, oxidation or phase transformation between organic matter and some types of surfaces are also possible70,134 but have not been systematically investigated until now. These future studies should consider the effect of light and O2, since these parameters can greatly influence the reactivity of DOM.250,255

dissolution. Some reactions may also occur between DOM and particle material and may result in release or consumption of ions. However, few studies addressed the effects of DOM on particle dissolution and a clear mechanistic understanding of these effects still lacks. Humic, fulvic and alginic acid have been reported to reduce the dissolution of Ag(0) nanoparticles coated with citrate and oleic acid.249,250 This observation was explained by the trapping of oxygenating species in humic acids matrix, inhibiting oxidation of Ag(0) to Ag+.250 These results apparently contradict other studies reporting an increase in dissolution of citrate stabilized Ag(0) particles in the presence of humic substances.250,251 Differences in size and shape of the used particles may explain the discrepancies between these results. Unrine et al. observed that DOM produced by plants stimulated the dissolution of Ag(0) nanoparticles coated with gum Arabic but stabilized silver particles coated with polyvinylpyrrolidone.252 Similar results were obtained for polyvinylpyrrolidone coated particles in the presence of humic acids.253 Thus, coating appears to be an essential parameter for dissolution processes. Concerning other materials, ZnO appears to dissolve faster in the presence of humic acids at basic pH, while at acidic pH no effect was observed.138 CdSe quantum dots dissolved faster in the presence of extracellular polymers, especially if these polymers contained proteins.254 Eventually, small chelating carboxylic acids increased dissolution of Cu/CuO nanoparticles.247 Interestingly, the dissolution of silver nanoparticles may be a reversible process. Indeed, several researchers have reported that humic substances can reduce silver and gold ions into stable Ag(0) or Au(0) nanoparticles under environmentally relevant conditions.250,255−258 In addition, DOM influence the kinetics and thermodynamics of nanoparticle formation as observed for systems like iron oxides or HgS.259,260 DOM also plays an important role for formation of a colloidal phase of hardly soluble heavy metal minerals in lead contaminated soils, which was explained by adsorption of organic material on the crystallite surfaces.261,262 The hydrophilisation of hydrophobic particles is another type of aging directly connected to the organic coating. DOM can modify hydrophobicity by replacing or overcoating the hydrophobic capping agent. Navarro et al. observed this phenomenon with CdSe quantum dots and different HfxZr1−xO2 crystals.140−142 Other types of aging are probably influenced by the presence of DOM and still need to be investigated in detail. For instance, oxidation, sulfidation or phase transformations are important

6. OUTLOOK Obviously, a major amount of knowledge has been accumulated in the last decades and a basic understanding of the interactions of DOM with colloids was gained. However, a systematic understanding of these highly complex systems will need further efforts. At first, future studies should systematically include a thorough characterization of DOM (molecular weight, molecular structure including configuration of reactive groups) and colloid (size, aggregation state, surface composition, etc.). This will help in elaborating accurate adsorption mechanisms of DOM. Competition between different kinds of DOM (humic substances, polysaccharides, proteins, fatty acids, etc.) for sorption on colloid surfaces still requires further attention in order to know if one type of DOM dominates the interactions with colloids. This point is essential for understanding the fate of colloids then natural surface waters often contains a mixture of all types of DOM. Efforts are also needed for characterizing the structure and property of aggregates in environmental media in a more systematical manner in order to discriminate between aggregates formed by electrostatic destabilization and by bridging mechanism. Advanced analytical methods should be developed for that purpose.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 (0)6341 280-31571; e-mail: [email protected]. Notes

The authors declare no competing financial interest. 8955



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ACKNOWLEDGMENTS We kindly acknowledge Veronika Müller for her help during the systematical analysis of literature.DFG program INTERNANO, subproject MASK SCHA 489/16.

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