Adsorption of gold nanoparticles on illite under high solid/liquid ratio and initial pH conditions

Abstract

Adsorption of nanoparticles on minerals affects the fate and transport of nanoparticles directly and is of great significance to many fields, including research into ore deposits, geochemistry, the environment and mineral materials. Whereas many previous studies have been conducted under the equilibrium pH and low solid (mineral) to liquid (nanoparticle suspension) ratio conditions, adsorption processes under initial pH and high solid/liquid ratio conditions may represent many important yet underexamined complex scenarios. To fill in this research gap, the adsorption of gold nanoparticles on illite was investigated experimentally at a relatively high solid/liquid ratio of 5 g L^–1 and the effects of initial pH, ionic strength, citrate concentration, temperature and illite particle size were evaluated. The adsorbed amount of gold nanoparticles (from <5% to nearly 100%) increased with increasing ionic strength, temperature and citrate concentration and decreased with increasing pH and illite particle size. The presence of illite resulted in the dynamic evolution of the pH of the suspension, which, along with solution chemistry parameters, controlled the electrostatic interaction of illite and gold nanoparticles. The adsorption results, scanning electron microscopy observations and surface properties of illite suggest that the negatively charged gold nanoparticles were adsorbed predominantly on the positive illite edges through electrostatic interaction. The electrostatic attraction between illite and gold nanoparticles appeared to be strong, supported by the minor amount of desorption. These research findings are expected to provide a valuable reference regarding many critical issues in the geosciences as well as for industrial applications.

During the last several decades, large quantities of engineered nanoparticles (ENPs) have been manufactured owing to their size-dependent properties and numerous applications in consumer and industrial products (Hendren et al., 2011; Sharma et al., 2019; Abbas et al., 2020a), and hence the potential release of ENPs into the land, soil, water, air and ecosystems and the associated impacts of this have attracted considerable attention (Nowack & Bucheli, 2007; Brar et al., 2010; Keller et al., 2013). Since Earth's origin, natural nanoparticles (NNPs) formed via various natural processes have been abundantly present, with the estimated annual flux of NNPs (hundreds of teragrams) to the Earth's surface and atmosphere being around three orders of magnitude greater than that of ENPs (Hochella et al., 2019). Given the distinctive behaviours of natural and anthropogenic nanoparticles (NPs) as well as their abundance and ubiquity in the Earth system, it has become increasingly recognized that NPs might play critical yet not entirely understood roles in many Earth processes, with fundamental implications possibly traversing multiple temporal and spatial scales (Hochella et al., 2002, 2008, 2019).

One of the most relevant such research topics addresses the interaction of NPs with their surrounding minerals in natural environmental media, specifically the adsorption of NPs on minerals (or heteroaggregation between NPs and minerals). As indicated by numerous previous studies, because the transport, exposure route, fate, distribution and bioavailability of NPs can all be substantially influenced by the adsorption of NPs on minerals, understanding such adsorption behaviour is desirable in various scientific fields, including but not limited to ore deposit formation (Reich et al., 2005; Hannington et al., 2016; Zhou et al., 2017), explorative geochemistry (Cao & Cheng, 2020; Hu et al., 2020), environmental science (Novikov et al., 2006; Sharma et al., 2015; Hochella et al., 2019), ecology (Buzea et al., 2007; Hochella et al., 2008; Wang et al., 2019), nanotoxicity (Borm et al., 2006; Amde et al., 2017; Abbas et al., 2020b), catalysis (Bond & Thompson, 2006; Wang et al., 2014) and analytical science (Alvarez-Puebla et al., 2005; Sathuluri et al., 2011; de Barros et al., 2017). For example, in the field of ore deposits, the transport and adsorption of metallic NPs on host minerals has long been proposed as an effective metal enrichment mechanism for several important hydrothermal and supergene deposits (Frondel, 1938; King et al., 2014; Saunders & Burke, 2017; Petrella et al., 2020, 2022; Saunders et al., 2020; McLeish et al., 2021; Wierchowiec et al., 2021), whereas the geochemical anomalies detected in the overburden and that are useful in locating buried economic mineral deposits can result from the natural dispersion of indicator element-bearing NPs and therefore should be influenced by the adsorption/deposition of NPs (Wang et al., 2016; Reith & Cornelis, 2017; Zhang et al., 2019). In environmental science, an ongoing concern is how and to what extent nanotechnology will impact the environment, organisms and human health, and various transformations of NPs (especially ENPs), including adsorption, agglomeration and chemical reactions in various environment compartments, have been demonstrated to change significantly their mobility, exposure form, bioavailability and toxicity (Tiede et al., 2009; Wang et al., 2019; Abbas et al., 2020a, 2020b; Spurgeon et al., 2020).

To date, several investigations have been performed to understand the adsorption behaviour of NPs (metals, metal oxides, graphene oxides, etc.) on minerals (pyrite, goethite, quartz, montmorillonite, kaolinite, diatomite, etc.), unveiling a complex adsorption process coregulated by a variety of factors including the physicochemical characteristics of NPs/minerals (composition, structure, morphology, surface charge, etc.) and solution conditions (pH, ionic strength (IS), electrolyte, organic matter, temperature, etc.) (Mikhlin et al., 2007; Zhou et al., 2012; Labille et al., 2015; Wang et al., 2015b, 2019; Zhao et al., 2015; Fu et al., 2017; Luo et al., 2018; Syngouna et al., 2018; Lu et al., 2019; Dong & Zhou, 2020; Li et al., 2020). By combining the effects of van der Waals attraction, electric double-layer (EDL) forces and perhaps additional interactions, the classical and extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theories serve well as useful frameworks to account for observed aggregation and adsorption behaviours between colloidal particles, at least in a semiquantitative manner (Derjaguin & Landau, 1941, Verwey & Overbeek, 1948; Thio et al., 2010; Wang et al., 2015a). Yet the quantitative explanation and prediction of such behaviours are still lacking (Hotze et al., 2010; Petosa et al., 2010; Liu et al., 2013, 2014), and sometimes the discrepancies between theories and observations can reach several orders of magnitude (Elimelech et al., 1995; Petosa et al., 2010). Such serious disparities, which can be attributed to the challenges stemming from the small size and special structure of NPs and the surface charge heterogeneities of minerals, not only demonstrate the inadequacy of the DLVO theories to quantify NP–mineral interactions accurately, but also warrant experimental endeavours to gather basic adsorption data.

Analytical approaches of counting and sizing NPs, such as dynamic light scattering (DLS) and time-resolved laser diffraction, have often been employed to acquire valuable information on aggregation rate and attachment efficiency. Despite the proven potency of such methodologies in heteroaggregation studies, their general limitation to dilute suspensions and intrinsic overestimation of larger-sized mineral particles cannot be overlooked (Zhou et al., 2012; Labille et al., 2015; Liu et al., 2015; Wang et al., 2015b, 2019; Gallego-Urrea et al., 2016; Praetorius et al., 2020). Although many laboratory experiments have been carried out successfully under pre-adjusted pH conditions to obtain equilibrium adsorption results (Labille et al., 2015; Liu et al., 2015; Wang et al., 2015b, 2019, 2021; Huang et al., 2016; Syngouna et al., 2018; Tang & Cheng, 2018; Lu et al., 2019; Dong & Zhou, 2020; Guo et al., 2020), we argue that such pre-adjustment of pH is not a necessary step in many important natural and even some engineering processes. Instead, real-world scenarios often involve interaction (or mixing) between a substantial amount of minerals (high solid/liquid ratio) and a NP-containing fluid with an initial pH. For example, as mentioned earlier, the interaction of a metallic NP-bearing fluid (having an initial pH) with wall rock may be critical to the formation of certain ore deposits (e.g. Carlin-type gold deposits), while the rock/fluid ratio can be as high as 5 g L^–1 (Kusebauch et al., 2018; Kusebauch et al., 2019). The suspended particulate matter (SPM) of some rivers, into which release of ENPs is possible, can reach up to tens of grams per litre (e.g. during floods; Thill et al., 2001; Slomberg et al., 2017). In sewage treatment plants, activated sludge is used to remove NPs from wastewater at a solid/liquid ratio of a few grams per litre (Barton et al., 2014; Chen et al., 2014), whereas in soil remediation sites, NPs are used to remediate contaminated soil at a solid/liquid ratio of hundreds of grams per litre (Machado et al., 2013; Zialame et al., 2021). It should be noted that dynamic evolution from initial to final pH of the mixed suspensions is expected in most of the above cases due to reactions between protons and minerals, especially at high solid/liquid ratios.

To the best of our knowledge, there are very few systematic investigations of the adsorption of NPs on minerals under conditions of a high solid/liquid ratio and an initial pH, which may simulate accurately several important processes and applications. Therefore, in this work, we carried out an experimental study of gold NP (AuNP) adsorption with illite under the conditions of a relatively high solid/liquid ratio (5 g L^–1) and initial pH values of 4–10. AuNPs were chosen as representative metallic NPs mainly because of their confirmed occurrences and potentially important roles in a number of ore deposits (e.g. at the rim of clay mineral grains in weathering deposits) and in some surficial environments (Bakken et al., 1989; Hong et al., 1999; Palenik et al., 2004; Reich et al., 2005; Hough et al., 2008, 2011; Southam et al., 2009; Lintern et al., 2013; Saunders & Burke, 2017; Zhou et al., 2017; Wierchowiec et al., 2021). Clay minerals are important components of sediments and soils that are distributed widely in supergene environments and critical zones (Bergaya & Lagaly, 2013; Hochella et al., 2019), and they show great potential in agricultural, industrial and medicinal applications (Bedelean et al., 2009; Floody et al., 2009; Martin et al., 2018). As one of the most common and major potassium clay minerals in the surface environment, illite (a dioctahedral 2:1 phyllosilicate) accounts for more than half of the total clay minerals in the Earth's crust (Gradusov, 1974). We thus selected illite to represent the widespread clay minerals in the environment, which, given their ubiquity, are very likely to interact with NPs and affect the fate and transport of NPs significantly (Zhou et al., 2012; Sotirelis & Chrysikopoulos, 2016; Yu et al., 2019, 2020). Furthermore, the patch-wise charge heterogeneity and great specific surface area of clay minerals may have a profound influence on the adsorption behaviour of AuNPs. To the best of our knowledge, only a few experimental studies have been reported on the adsorption of AuNPs on clay minerals. Among them, Gallegot-Urrea et al. (2016) tested the aggregation rate of AuNPs (80 μg L^–1) under simulated natural freshwater conditions using DLS. Possibly due to the relatively low concentration of illite (650 μg L^–1), illite did not promote the agglomeration of AuNPs significantly (Gallego-Urrea et al., 2016). By evaluating the distribution and adsorption behaviour of AuNPs in soil, Reith et al. (2017) found that clay minerals and organic matter in soil were the most important adsorbents of AuNPs. Yet the influence of the medium conditions on the adsorption of AuNPs by a single adsorbing component (e.g. a clay mineral) in the soil was not investigated specifically (Reith et al., 2017). Our experimental investigation of AuNP adsorption on illite under various solution conditions has generated systematic data and findings that we believe will provide new insights into the governing mechanisms of AuNP–illite adsorption and shed light on the role of NPs in aquatic environments.

Materials and methods

Reagents

Chloroauric acid tetrahydrate (HAuCl₄⋅4H₂O; ≥99.9%) was purchased from Shanghai Jiuyue Chemical Corporation (Shanghai, China). Trisodium citrate (≥99.9%) was purchased from Shanghai Shenbo Chemical Corporation (Shanghai, China). Hydrochloric acid (HCl; 36–38%), nitric acid (HNO₃; 65–68%), sodium chloride (NaCl; ≥99.0%) and sodium hydroxide (NaOH; 99.0%) were purchased from Sinopharm Chemical Reagent Corporation (Shanghai, China). All solutions were prepared using deionized water (Millipore Corporation, Molsheim, France) with a specific resistivity of 18.2 MΩ⋅cm. All glassware was cleaned and soaked in aqua regia (HCl/HNO₃ = 3:1, v/v) for at least 8 h, before a thorough rinse with deionized water followed by oven drying at 50°C for 48 h.

Synthesis and characterization of AuNPs

Monodisperse suspensions of negatively charged AuNPs with a particle size of ~18 nm were synthesized using the Frens method (Frens, 1973), in which sodium citrate was used as a reducing agent and stabilizer. Briefly, 300 mL of HAuCl₄ solution (0.01%, w/w) was first heated to boiling. Then, 10.5 mL of sodium citrate solution (1.00%, w/w) was added rapidly, and the boiling was continued for 20–30 min to complete the reaction. After the reaction, a wine-red suspension of AuNPs was obtained, which was cooled to room temperature and then stored in a refrigerator at 4°C. The Au concentration of the as-synthesized suspension measured using atomic absorption spectroscopy (AAS; 990SUPER, Persee, Beijing, China) was ~57.6 ppm. The particle size and ζ-potential of AuNPs were determined using a multi-angle particle sizer and high-sensitivity ζ-potential analyser (Omni, Brookhaven Instruments, New York, NY, USA), respectively. Only negligible changes in particle size and ζ-potential (Fig. S1) were found after 5 days of storage (greater than the adsorption period), indicating that our AuNP suspensions were relatively stable.

Illite sample characterization

The illite sample was collected from Jinsha, Guizhou Province, China. A fraction of illite particles <2 μm was obtained through washing, crushing, milling and sedimentation. The crystal structure of illite was verified using power X-ray diffraction (XRD; Empyrean, Eindhoven, The Netherlands) analysis, which was conducted with Cu-Kα radiation operated at 40 kV and 40 mA, and with a scan of 5°–60°2θ (Fig. S2a). The major element composition was determined using X-ray fluorescence (XRF; ARL Perform'X 4200, Thermo Fisher, Waltham, MA, USA) spectrometry (Table S1). Because of the similar XRD features of illite and muscovite, thermal analysis (STA 449 F3 Jupiter, Selb, Germany) and XRF were used to distinguish the two minerals. In comparison with muscovite, the lower dehydroxylation temperature (Fig. S2b), lesser potassium content (Table S1) and the XRD analysis results confirmed that our sample was high-purity illite (Gaines et al., 1964; Mackenzie et al., 1966; Schomburg et al., 1997). The typical scale-like surface morphology of the illite sample was characterized using scanning electronic microscopy (SEM; Scios, Hillsboro, OR, USA; Fig. S3) with an acceleration voltage of 30 kV. The electrokinetic potential and the point of zero charge (pH_PZC; ~5.8) of the illite sample were determined using a ζ-potential analyser (Omni, Brookhaven Instruments, New York, NY, USA) and an automatic potentiometric titrator (Metrohm 905, Herisau, Switzerland), respectively (Fig. S4).

Batch adsorption experiments

Batch adsorption experiments were conducted under controlled solution chemistry conditions to examine the effects of pH, IS, natural organic matter (NOM) and temperature on the adsorption behaviour of AuNPs on illite at a constant solid/liquid ratio of 5 g L^–1. All batch experiments were performed in a constant-temperature shaker and were run at least in duplicate. The initial pH of each AuNP suspensions was adjusted in the range of 4–10 using 2 M HCl or NaOH solution. The concentration of sodium citrate in AuNPs suspensions was varied in the range of 1.0–7.5 mM to investigate its effect on adsorption behaviour, with the background concentration of sodium citrate from the as-synthesized AuNP suspension determined to be ~1 mM. The ISs of the Au colloids were controlled by adding various amounts (0, 5 or 10 mM) of NaCl to the suspensions. For a typical adsorption experiment, 0.2 g of illite and 40 mL of 57.6 ppm AuNP suspension were added into a 100 mL conical flask and shaken for 5 days at various temperatures (5, 25 or 60°C). Subsequently, 3–5 mL of the mixed suspension was sampled and centrifuged for 30 min (3000 rpm). Then, 1 mL of the supernatant was collected and digested (1 mL aqua regia, overnight) to analyse the Au concentration using AAS. The sodium citrate concentrations of the suspensions before and after adsorption were determined using high-performance liquid chromatography (HPLC; Agilent 1200, Santa Clara, CA, USA). The ζ-potential and dissolved cation concentrations of illite under various solution chemistry conditions were determined using a ζ-potential analyser (Omni, Brookhaven Instruments, New York, NY, USA) and inductively coupled plasma optical emission spectrometry (ICP-OES; VISTA-MPX, Varian, Palo Alto, CA, USA), respectively. All of the tests were performed at least in duplicate. Based on the measured concentration of Au or sodium citrate, the adsorption amount (q_t) and adsorption percentage (%R) were calculated according to Equations 1 and 2:

(1)$${q}_{t} = {V}{c}_ 0{( 1\;}-{\;}\displaystyle{{C} \over {{C}_ 0}}{) /m}$$

(2)$${\% R} = {( 1\;}-{\;}\displaystyle{{C} \over {{C}_ 0}}{) \;\times \;100\% }$$

where c is the concentration of the supernatant at time t, c ₀ is the initial concentration of the solution, ${{C} \over {{C}_ 0}}$ is the relative residual (unadsorbed) concentration of Au or sodium citrate in the supernatant, V is the volume of the solution and m is the mass of the illite sample.

The desorption experiments were conducted by first separating illite (centrifugation: 3000 rpm, 30 min) from the sorption suspensions and then shaking it in 40 mL of deionized water or 1 mM sodium citrate solution (similar to the background concentration in the as-synthesized AuNP suspension) at pH 4. As with the adsorption experiments, after desorption, the concentration of Au in the supernatant was measured using AAS.

Results and discussion

Effect of pH on the adsorption of AuNPs on illite

As the surface electrical properties of both AuNPs and illite will change with varying pH (Cottet et al., 2014; Rawat et al., 2018), the adsorption behaviour of AuNPs with pH as a master variable was first investigated. The initial pH range of AuNP suspensions was chosen to be 4–10, which is close to the pH range of natural water (Huertas et al., 2001). As indicated in Fig. S1, within the whole experimental pH range, all of our AuNPs were negatively charged (due to complexation with negatively charged citrate groups) and all suspensions remained stable for over 5 days. For illite as a typical 2:1 clay mineral, the substitution of Si⁴⁺ and Al³⁺ in the tetrahedral and octahedral sheets by lower-valence cations creates permanent negative charges on the basal planes, whereas pH-dependent charges develop at the edges of the illite planes where the Si–Al lamellar structure breaks (Avena, 2003; Delhorme et al., 2010; Liu et al., 2013). At pH values below the pH_PZC (measured to be 5.8), the basal plane of illite is negatively charged, whereas the illite edge is positively charged due to the protonation of the amphoteric Si–OH and Al–OH groups. At pH values greater than the pH_PZC, both the illite edge and basal plane are negatively charged. It is worth noting that at a relatively high solid/liquid ratio (5 g L^–1), a change in the system pH was anticipated due to the interlayer cation exchange of illite and the protonation or deprotonation of the hydroxyl groups on the edge face of illite. As shown in Fig. S5, when the initial pH was changed from 4 to 7, the suspension pH eventually rose from 4.9 to 8.1, and the decrease of pH from 10.0 to 8.4 was due to the influence of atmospheric carbon dioxide.

Figure 1a shows the relative concentrations of AuNPs in the supernatant as a function of adsorption time and initial pH. It can be seen that all adsorption experiments at various initial pH values almost attained equilibrium when the adsorption time reached ~30 h, and the adsorption amount decreased with increasing pH. The maximal adsorption (~70%) of AuNPs on illite occurred at the lowest initial pH of 4. At pH > 5, only <10% of AuNPs were adsorbed on illite after 50 h of adsorption, whereas insignificant adsorption of AuNPs was observed at a pH > 7. Such adsorption behaviour is explained through electrostatic interaction between the negative charge of AuNPs and the patch-wise charge on illite. At pH < pH_PZC of illite, although the overall electrical interaction between AuNPs and illite (due to its dominant negative charge at basal planes; see Fig. S4a) is still repulsive, the positive charges (i.e. protonated hydroxyl groups) on illite edges provide attractive adsorption sites for negatively charged AuNPs (see Fig. S1a). More importantly, our experiments imply that when AuNPs migrated to the vicinity of the illite edge and possibly with a proper orientation, the repulsion between AuNPs and illite can be overcome and the resulting local net attraction can lead to the favourable adsorption of AuNPs. The adsorption amount would depend on both the repulsive and attractive forces between AuNPs and the heterogeneous structure of illite. Thus, the greatest adsorption took place at pH 4 (among the pH range of 4–10), when the repulsion appeared to be the least (due to lower absolute ζ-potential values for both AuNPs and illite; see Figs S1a & 4a) and the attraction might be greatest (due to greater positive charge at the illite edge; see Fig. S4b). Similarly, when pH increased, the repulsion became stronger while the attraction diminished gradually, leading to a lesser adsorption extent. Finally, when the system pH (e.g. initial pH 7–10) was significantly greater than the pH_PZC of illite, the attraction force for AuNPs at the negatively charged illite edge disappeared, and then the AuNPs adsorption became negligible.

Figure 1. (a) Relative residual concentration of AuNPs in the supernatant at various initial pH values as a function of adsorption time. (b) Relative residual concentration of citrate in the supernatant at various initial pH values as a function of pH adsorption time. (c) Adsorbed AuNPs and citrate on illite at various initial pH values.

The adsorption behaviour of citrate alone (c ₀: 1 mM, similar to the concentration of the as-synthesized AuNP suspension) on illite was further compared as a function of adsorption time and pH. Figure 1b shows that the adsorption amount of citrate on illite decreased with increasing pH, consistent with the adsorption trend of AuNPs. As citric acid is a triprotic carboxylic acid, the citrate molecule was always negatively charged under our experimental conditions, and its speciation and average charge depended on the pH of the solution. In general, the adsorption behaviour of organic acids can be explained by considering the electrostatic interactions between the carboxylic acid species and the charged surface sites of clay minerals (Ward & Brady, 1998). Similar to the AuNP adsorption mechanism discussed above, a greater pH induces greater repulsion and less attraction between citrate (with more deprotonated species; Ramos & Huertas, 2014) and illite (with diminishing positive charges at edge faces), resulting in a reduced adsorption amount of citrate at greater pH values. Such a similar adsorption trend with pH confirms the importance of citrate dissociation on the surface of AuNPs as well as the heterogeneous charging behaviour of illite, therefore reinforcing the related electrostatic mechanism in the adsorption process. Despite the apparent resemblance between AuNP and citrate adsorptions on illite, the percentage adsorption of AuNPs is generally lower than that of citrate (Fig. 1c), which is attributed to the greater repulsion between illite and AuNPs complexed with many citrate surface ligands. As the maximum adsorption amount of AuNPs on illite occurred at pH 4, we will set the initial pH of the suspension to 4 to illustrate the influences of IS, sodium citrate concentration and temperature on the adsorption process in the following sections.

Effect of IS on the adsorption behaviour of AuNPs

IS is known to affect the electrostatic potential of charged particles such as NPs and clay minerals and thus to affect their aggregation and adsorption behaviours (Rawat et al., 2018). Generally, an increase in the solution IS leads to a decrease in both the range and magnitude of the EDL interactions. To elucidate the effect of IS on the adsorption process, we conducted adsorption experiments with controlled IS (through the addition of 0, 5 or 10 mM NaCl) at pH 4 and at a constant solid/liquid ratio of 5 g L^–1. As shown in Fig. 2a, the adsorption rate and adsorption amount of AuNPs on illite increased significantly with the addition of NaCl, in agreement with the similar impact of ζ-potential found in previous studies (Saka et al., 2006; Lu et al., 2019). At a concentration of 10 mM added NaCl, ~94% of AuNPs were removed from the suspension within just 0.5 h. Such a sharp influence of IS on the adsorption process can be explained in terms of the following aspects: first, in consideration of the patch-wise charge heterogeneity of illite, the spillover of the illite basal plane EDL to the edge is substantial at low IS and will to some extent shield the edge EDL, thus creating a strong repulsive energy barrier to the approaching AuNPs. At an increased IS, the EDL on the basal plane becomes sufficiently compressed (Fig. 2b) and therefore the attractive EDL on the edge will emerge as a more influential force for the adsorption of more AuNPs (Tombácz & Szekeres, 2004; Zhou et al., 2012). Second, an increase in IS also screens the electrostatic repulsions amongst charged AuNPs, making them less stable and more prone to homoaggregation. Such a promoted homoaggregation of AuNPs by increased IS was also observed in our previous investigations (Luo et al., 2018).

Figure 2. (a) Effect of IS (addition of 0, 5 or 10 mM NaCl) on the adsorption of AuNPs on illite. (b) ζ-potential of illite under various IS conditions.

Effect of citrate concentration on AuNP adsorption

As discussed above, citrate exists as a surface ligand that stabilizes AuNPs, and its concentration in our as-synthesized suspension was ~1 mM. As a low-molecular-weight organic anion, citrate has been found in natural environments, including soil, sediments and aerosols, and it can originate from the metabolic processes of plant roots and microorganisms (Ryan et al., 2001; Braissant et al., 2002). In addition, citrate is often used as a reducing and stabilizing agent in the process of ENP synthesis, and it can be released with ENPs and influence on ENP transformations (Wagener et al., 2012). Therefore, there is a need to explore the effects of citrate as a representative organic compound on the adsorption of AuNPs on illite. For organic molecules in NP suspensions, both stabilization (due to electrosteric forces) and destabilization (due to bridging) roles have been recorded experimentally, and it is often difficult to predict these interactions (Wang et al., 2015a; Gallego-Urrea et al., 2016; Tang et al., 2018; Ahmed et al., 2021). Compared with many other natural organic compounds, such as humic acid, citrate has the advantages of a well-defined and much simpler chemical structure, potentially facilitating the interpretation of its role in adsorption mechanisms.

In this work, the citrate concentration was increased from the original concentration of 1 mM in the as-synthesized AuNP suspension to an upper limit of 7.5 mM. It can be seen from Fig. 3a that the adsorption of AuNPs generally increased with increasing citrate concentration. At a citrate concentration of 7.5 mM, nearly full adsorption of AuNPs on illite could be achieved, which is significantly greater than the 70% AuNP adsorption at a citrate concentration of 1 mM. In addition, the time needed to reach the equilibrium c/c ₀ value decreased with increasing citrate concentration, indicating an increased AuNP adsorption rate after citrate addition.

Figure 3. (a) Effect of citrate concentration (1.0–7.5 mM) on the adsorption of AuNPs. (b) ζ-potential of AuNPs at various citrate concentrations. (c) The adsorption of citrate on illite at various citrate concentrations (without AuNPs). (d) ζ-potential of illite at various citrate concentrations (without AuNPs).

In general, the stability of NPs in a suspension increases with increasing ligand concentration due to enhanced surface complexation. As shown in Fig. 3b, the ζ-potential of our negatively charged AuNPs did become more negative with increasing citrate concentration, suggesting greater citrate coverage on the surface of the AuNPs (Wagener et al., 2012). Similarly, the measured adsorption of citrate on illite (Fig. 3c) and thus the absolute value of the illite ζ-potential (Fig. 3d) also increased after citrate adsorption. Such more negative ζ-potential values of both AuNPs and illite should result in greater electric repulsion between particles and contribute to less adsorption of AuNPs. Additionally, the competitive adsorption of citrate on illite edges could also lead to reduced AuNP adsorption. The above mechanisms, although supported by many previous studies (Yang et al., 2013; Wang et al., 2015b; Dong & Zhou, 2020; Guo et al., 2020), do not agree with the observed increased AuNP adsorption at greater citrate concentrations in our work, suggesting that some other role of citrate must be contributing to the improved AuNP adsorption. The well-known destabilizing bridging role is considered minor for citrate in our experiments, as bridging between particles normally occurs with larger organic molecules at relatively low concentrations (Philippe & Schaumann, 2014; Yu et al., 2018; Ahmed et al., 2021). Under high solid/liquid ratio and initial pH conditions, the pH buffering effect of citrate (a tricarboxylic acid anion) deserves attention because pH is a decisive factor in adsorption, and the presence of a large amount of illite is known to cause considerable change in the pH of a system (see Fig. S5). As shown in Fig. S6, the addition of citrate reduced the increase in pH from the initial value (pH 4) to the final value. For example, the final pH became ~4.2 when the citrate concentration was 7.5 mM, compared with the final pH of 4.9 at the original citrate concentration of 1.0 mM. Although this buffering effect may seem to have caused only a small difference in the final pH, the resulting lower pH (at greater citrate concentrations) should be sufficiently favourable to create more positively charged sites at the illite edge (see Fig. S4b), enabling the adsorption of more AuNPs (compared with the effect of an initial pH of 4.0–4.5 presented above). Additionally, an increase in citrate anion concentration increases the IS of the system, which, as shown above, can screen the electrostatic repulsion between negatively charged particles and thus promote their (hetero)aggregation (Johnson & Lenhoff, 1996; Sadowska et al., 2014). Finally, the addition of citrate can facilitate the dissolution of illite (Table S2), which not only significantly increases the IS of the suspension, but also releases some high-valence framework ions, having a strong effect on aggregation according to the empirical Schulze–Hardy rule (Yecheskel et al., 2019; Dong & Zhou, 2020) and therefore promoting AuNP adsorption substantially.

Effect of temperature on the adsorption process

Temperature can affect the thermodynamics and kinetics of adsorption processes profoundly (Freitas & Muller, 1998; Das, 2017). Figure 4a shows the overall effect of temperature on the adsorption of AuNPs on illite. Specifically, a significant increase in the adsorption rate during the initial hours of the experiment was found at greater temperatures, which is attributed to the known more rapid kinetics promoted by increased temperature. In addition, when the temperature of the system increased from 5 to 60°C, the equilibrium adsorbed amount of AuNPs on illite increased from ~70% to 95%. Such increased AuNP adsorption on illite with increasing temperature is explained by the following aspects: primarily, it is important to note that the absolute ζ-potential values of AuNPs (Fig. 4b) and illite (Fig. 4c) both decreased with increasing temperature. For example, when the temperature was increased from 5 to 60°C, the ζ-potential values of AuNPs and illite changed from ‒35 to ‒12 mV and from ‒12 to ‒1 mV, respectively. The reduced absolute ζ-potential value of AuNPs with increasing temperature was ascribed partly to the increased pK_a value of citric acid, which corresponds to the reduced extent of citric acid dissociation and therefore number of negative charges on the surface of AuNPs (Goldberg et al., 2002). In addition, as demonstrated in Table S3, increasing temperature also promoted the dissolution of illite (involving the release of high-valence framework cations), which increased the IS of the suspension and in turn contributed to decreasing the absolute ζ-potential values of both AuNPs and illite. The lowering of absolute ζ-potential values, which reduces the electrostatic repulsion, along with the release of high-valence cations that promote aggregations, made it easier for AuNPs to adsorb on the edges of illite. As shown in Fig. 5, numerous AuNPs were adsorbed on the edge face of illite at 60°C, which is consistent with our previous observations (Fu et al., 2020).

Figure 4. (a) Effect of temperature on the adsorption of AuNPs on illite. (b) ζ-potential of AuNPs at various temperatures. (c) ζ-potential of illite at various temperatures. (d) pH changes of illite in deionized water at various temperatures.

Figure 5. SEM images of illite after adsorption of AuNPs at 60°C: (a) secondary electron images, (b) backscattered electron images.

Effect of illite particle size on the adsorption process

Natural mineral particles exist in a range of particle sizes, and these varying sizes could play a role in the AuNP adsorption process. To explore the effect of illite particle size, illite with a greater particle size of ~74 μm was selected for use in adsorption experiments at pH 4–7. Compared with the aforementioned experiments with illite of smaller size (<2 μm), a similar trend in the variation of pH was displayed for the experiments with larger-sized illite particles (see Fig. 6a). That is, the adsorption of AuNPs always decreased with increasing pH. The difference is that the larger-sized illite particles appeared to adsorb fewer AuNPs than the smaller-sized illite particles under the same experimental conditions. At pH 4, compared with 70% AuNP adsorption on the smaller-sized illite, only 40% of AuNPs were adsorbed on the larger-sized illite, whereas at pH 6 (and higher) only <5% of AuNPs were adsorbed on the larger-sized illite (Fig. 6a). The ζ-potential measurements indicated that the larger-sized illite displays approximately two times more negative ζ-potential values than the smaller-sized illite across the whole pH 4–9 range (compare Fig. 6b with Fig. S4a). The reasons for the significant difference in AuNP adsorption between illite particles of various sizes may be twofold: first, the smaller-sized illite exposes more edge sites capable of adsorbing AuNPs; and second, because of there being relatively more positive charges on the exposed edge faces of the smaller-sized illite, the overall ζ-potential becomes less negative and thus exerts less electrostatic repulsion on the approaching AuNPs.

Figure 6. (a) Effect of pH on the adsorption of AuNPs on illite (~74 μm). (b) ζ-potential of illite (~74 μm).

Desorption of negatively charged AuNPs

Whether and to what extent NPs can desorb from mineral surfaces is important, in that desorption behaviour influences the fate and transport of NPs directly and also provides some useful insights into the adsorption mechanism. Thus, we carried out desorption experiments by first obtaining illite from the adsorption suspension (initial pH 4) through centrifugation, and then, with a constant solid/liquid ratio maintained at 5 g L^–1, adding deionized water (pH 4) with or without 1 mM citrate to the separated illite. The results of the desorption experiments are shown in Fig. 7. Up to ~9 ppm (slightly more than 20%) of the pre-adsorbed AuNPs were desorbed from illite after exposure to deionized water or sodium citrate solution, implying that the majority of the adsorbed AuNPs might be bonded firmly to the edge sites of illite. In other words, from an energy perspective, we believe that most AuNPs were deposited in the primary energy minima (instead of the secondary energy minima) according to DLVO theory, and the reversible release of such adsorbed AuNPs following changes in solution chemistry is considered improbable. Additionally, Fig. 7 also shows that the amount of AuNPs desorbed in water was greater than that in 1 mM citrate, which demonstrates inhibition of AuNP desorption due to the presence of citrate and is consistent with the result of citrate promoting AuNP adsorption as discussed above.

Figure 7. Au concentration in the supernatant after the desorption experiment.

Conclusion

We conducted a systematic study of the adsorption behaviour of negatively charged AuNPs on illite under conditions of relatively high solid/liquid ratios and initial pH. The adsorbed amount of AuNPs, which can vary widely from being negligible (much less than 5%) to nearly complete (up to 100%), was found to decrease with increasing pH or illite particle size and to increase with increasing IS, citrate concentration and temperature. Upon investigation of the effects of the above key factors, a number of complex processes and mechanisms have been identified that control AuNP adsorption behaviour, and the structural and charge heterogeneity of illite have been demonstrated also to play a substantial role. Under our experimental conditions, the suspension pH can be changed significantly with the presence of illite through interlayer cation exchange, edge face hydroxyl protonation and dissolution. The suspension pH and other solution chemistry parameters (IS, citrate concentration, etc.) modulate the electrostatic interaction between illite and AuNPs, which is considered to be the dominant mechanism in the AuNP adsorption process (as summarized in Fig. 8). At pH values greater than the pH_PZC, both the basal plane and edge face of illite are negatively charged, and the electrical repulsion between illite and negatively charged AuNPs is strong enough to prevent any significant adsorption. However, at pH values lower than the pH_PZC, the illite edge becomes positively charged, resulting in electrostatic attraction and considerable adsorption of AuNPs at the edge sites. The addition of salt to the suspension either directly or through the dissolution of illite (promoted by increased temperature or citrate concentration) can suppress the EDL repulsion between particles and thus facilitate the adsorption and aggregation processes. The pH buffering role of citrate is also proposed to contribute to the increased AuNP adsorption at greater citrate concentrations. Finally, our desorption experiments suggest that most adsorbed AuNPs are bonded strongly with illite, possibly in the primary energy trap. The results from this work are expected to provide new and valuable information applicable to many natural and engineering processes that involve migration and enrichment of elements in NPs. Based on the findings of this work, future studies could aim to further predict or optimize NP adsorption processes to expand the application of clay minerals and NPs in environmental management, catalysis, photoreactions, biomedicine and other fields.

Figure 8. Schematic representation of the mechanism of adsorption of negatively charged AuNPs under various conditions: (a) pH, (b) citrate concentration, (c) IS, (d) temperature.