les coated with a guanidinium-functionalized polyelectrolyte extend the pH range for phosphate binding †

Del University of Technology, Departm Maasweg 9, 2629 HZ Del, The Netherland Wetsus – European Centre of Excellen Oostergoweg 9, 8932 PG Leeuwarden, The N Wageningen University & Research, Labora 6708 WE Wageningen, The Netherlands. E-m † Electronic supplementary informatio synthesized PAH–Gu (Fig. S1), pictur suspensions (Fig. S2 and S3). Pseudo se pseudo second-order non-linear curve  10.1039/c7ta04054g Cite this: J. Mater. Chem. A, 2017, 5, 18476


Introduction
The uncontrolled discharge of phosphate-containing products as present in aqueous streams from agricultural and cosmetic sectors 1,2 has increased eutrophication processes, i.e. the rapid growth of aquatic algae in lakes and rivers. 3,4These processes contribute to an unbalanced aquatic ecology and to a decrease of water quality.Phosphate is therefore considered to be one of the most critical contaminants present in wastewater.Many countries have set a standard for the discharge of phosphate into water. 5For instance, the European Union recently regulated a maximum value of 0.07 mg P per L for rivers and 100 mg P per L for lakes to reduce the risk of eutrophication. 6In order to meet such strict requirements and to manage the high phosphorus demand at the same time, 7,8 the recovery of phosphorus from phosphate-contaminated aqueous media has been recognized as a challenging key strategy.For this purpose different technologies have been developed, including biological treatments, 9 membrane-based processes, 10,11 crystallization, 12,13 otation, 14 and adsorption-based processes. 15From this list of well-known techniques, adsorption processes have high potential.This is mainly related to their low operational costs, high efficiency, low energy consumption and their versatility to be applicable in different wastewater sources. 15mong candidates for phosphate adsorbents, iron oxides are considered to be highly promising. 16This is because of (1) their high selectivity to bind phosphate in the presence of competing anions and (2) their easy introduction in municipal wastewater treatment plants (WWTPs).Furthermore, a good adsorbent is identied by, amongst others, the available specic adsorption area.For this reason, a lot of attention is now paid to develop new nano-sized adsorbents, because of their high-surface-areato-volume ratio. 17Nanoparticles of iron oxide (Fe 3 O 4 NPs) fulll these conditions and even possess magnetic properties, making easy separations possible by using external magnetic elds. 18hosphate adsorption onto Fe 3 O 4 NPs occurs through an innersphere complex, due to the presence of surface hydroxyl groups. 16,19When the pH is lower than the point of zero charge (PZC), the surface of the iron oxide nanoparticles is positively charged, which promotes binding and surface adsorption of phosphate anions.The lower the pH, the more charge on the surface and therefore a higher binding capacity. 20,21However, at lower pH values the amount of phosphate anions decreases, as they are converted to phosphoric acid. 22This becomes signicantly below pH < pK a1 ¼ 2.1.The pH of water streams in WWTPs is typically 6-8, 23,24 i.e., around the PZC of the Fe 3 O 4 NPs.At such pH values, the surface charge is slightly positive, neutral or slightly negative, which has a large negative impact on the phosphate anion binding capacity.In the mentioned pH range, the phosphates are monoanionic and partly dianionic (pK a2 ¼ 7.2). 22Moreover, in this pH range the NPs aggregate to precipitate, due to the decreased inter-particle electrostatic repulsions.Thus, for phosphate separation processes at pH values around the PZC of Fe 3 O 4 NPs, there is room for improvement.For this reason, different types of chemical surface modications have been applied by the attachment of specic ligands, including amino groups, 25 metal organic frameworks (MOFs), 19 polymers, 26,27 layered double hydroxides (LDHs) 28 and graphene. 29These examples illustrate well the effectiveness of surface functionalization in terms of controlling the affinity for a specic target species.Yet, it would be interesting to further employ these surface modication strategies in order to extend the use of iron oxide nanoparticles for phosphate anion binding at higher pH values, where unmodied iron oxide is otherwise less effective.
Receptor-functionalized polyelectrolytes (PEs) can bind to surfaces of opposite charge [30][31][32] and can contribute to nanoparticle stabilization, 33 while the receptor groups introduce selectivity for binding certain targets.Recent advances in this direction resulted in the availability of polyelectrolytes that were functionalized with, e.g., biotin, uorescent probes and guanidinium groups to address chelation and the selective capture of His-tagged proteins, 34,35 biosensing, 36 ngermark visualization, 37 and ion selectivity. 38Interestingly, polyelectrolyte functionalization and the subsequent modication of NPs do not require complicated chemical steps and can be performed rapidly in aqueous media.
In the current study, we present the concept of a simple surface modication of commercially available Fe 3 O 4 NPs using polyelectrolytes functionalized with phosphate-receptors.For the receptor we have chosen the guanidinium moiety, which is able to coordinate phosphate ions in a wide range of pH values. 39,40The Gu-functionalized polyelectrolyte was applied to modify the Fe 3 O 4 NPs.The thus-obtained NPs are characterized in terms of their morphology, thermal stability and surface properties.The effect of the pH on the phosphate adsorption is investigated in detail, as well as the kinetics of the process.The obtained results were compared with those of bare Fe 3 O 4 NPs as well as Fe 3 O 4 NPs coated with a non-functionalized polyelectrolyte.
Preparation of PAH-Gu, Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu PAH-Gu (Scheme 1, top right) was obtained by the reaction of GAA with a part of the amino groups of PAH following the procedure published earlier by our group. 38 1H-NMR was used to conrm the chemical structure (ESI, Fig. S1 †) and to calculate the degree of amino group functionalization by guanidinium (Gu) moieties; it was found to be $30% for the batch used in the current work.In order to study the effect of present Gu groups, non-functionalized PAH was used as a reference.
Next, the Fe 3 O 4 NPs were modied with PAH or PAH-Gu via the following procedure. 41Aqueous solutions of PAH and PAH-Gu (2.5 g L À1 ) were prepared by sonication using a probe sonicator (Cole-Parmer CPX750, 30% power, 750 watts) for 20 min and simultaneous cooling by placing the tube in ice.Similarly, a Fe 3 O 4 NP suspension (0.5 g L À1 ) was prepared in MilliQ water and sonicated under the same conditions.Aer sonication, the pH of all solutions was adjusted to 9.5 by the addition of drops of concentrated HCl or NaOH (1 M).At this pH value the Fe 3 O 4 NPs have a negative surface charge, while the polyelectrolytes are positively charged.The Fe 3 O 4 NP suspension was added drop-wise to the polyelectrolyte solutions and stirred for 24 h at room temperature (RT) to ensure complete adsorption at the Fe 3 O 4 NP surface.The functionalized NPs were separated from the excess of PEs by three cycles of centrifugation, decantation and washing (Heraeus instrument D-37520 Osterode, Germany) at 17 000 rpm (20 min at 20 C).The product was nally redispersed in 40 mL of MilliQ water to maintain the initial concentration and then sonicated to obtain uniform solutions of Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu.A schematic overview of the coating procedure and the different types of Fe 3 O 4 NPs is presented in Scheme 1.

Characterization
The 1 H NMR spectrum of the PAH-Gu polymer was obtained using a Bruker AVANCE 400 NMR spectrometer with D 2 O as solvent.
Modied NPs were studied with Fourier Transform InfraRed spectroscopy (Nicolet 8700 FT-IR Spectrometer) by mixing the NPs with KBr and pressing pellets.The spectral range of FT-IR was from 4000 cm À1 to 500 cm À1 with a resolution of 4 cm À1 .
X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientic, K-Alpha model) was used to determine the atomic composition of the modied NP surfaces.In more detail, a monochromatic Al Ka X-ray source was used with a spot size of 400 mm at a pressure of 10 À7 mbar.A constant pass energy of 200 eV for the survey spectra and 50 eV for the detailed highresolution spectra was used.The ood gun was turned on during the measurement to compensate for potential charging of the surface.The peak position was adjusted based on the internal standard C1s peak at 284.8 eV, with an accuracy of AE0.05 eV.Avantage processing soware was used to analyze all spectra.
ThermoGravimetric Analysis (TGA) measurements were performed with Thermal Analysis (TA) Instruments equipment from RT to 550 C at a heating rate of 10 C min À1 under continuous air purging.
The size and morphology of the unmodied and modied NPs were studied using a Transmission Electron Microscope (TEM, JEOL JEM-1400 Plus, USA) operated at 120 kV.A holey carbon support lm (200 meshes, Quantifoil®) was dipped into the NP-containing solution and then dried at room temperature overnight.TEM images were analyzed by using Image J soware and the mean size value of each NP system was calculated based on 20 separately determined diameters.
The hydrodynamic diameter (D h ) of NPs was determined at 25 C by Dynamic Light Scattering (DLS) using a Zetasizer Nano ZS900 (Malvern, UK).The instrument was operated at a backscattering angle of 173 with a laser beam with a wavelength of 633 nm.The same instrument was used to measure the z-potential at 25 C for all samples and measurements were performed in triplicate.To this end, an aqueous suspension of Fe 3 O 4 NPs (0.5 mg mL À1 ) was prepared by adding 167 mL of the original concentrated NP solution into 10 mL MilliQ water.Samples for z-potential measurements were prepared by diluting 80 mL of the above-prepared NP suspension (0.5 mg mL À1 ) to 10 mL using MilliQ water.The solution was sonicated using a probe sonicator (30%, 750 watts, cooling in an ice bath, 6 min) to break the existing aggregates.In the last step, the pH was adjusted to the desired values by using 1 M NaOH and 1 M HCl.The same procedure was used to determine the zpotential of all NP systems, as well of the pure PEs (PAH and PAH-Gu), where a solution of 0.5 mg mL À1 in MilliQ water was used.All the measurements were done 5 min aer the sonication procedure to minimize possible differences due to colloidal instability.

Batch adsorption experiments
Phosphate adsorption experiments were performed for Fe 3 O 4 , Fe 3 O 4 @PAH, and Fe 3 O 4 @PAH-Gu NPs.All desired phosphate solutions, including the standard known concentration of phosphate for calibration measurements, were prepared by diluting a stock solution (1000 mg L À1 of NaH 2 PO 4 in 250 mL).The phosphate adsorption was studied as a function of time starting with an initial phosphate concentration of 2 mg L À1 , taken from the stock solution, and an adsorbent solution of 0.5 g L À1 in 30 mL.The adsorbed amount was deduced from the reduction of the phosphate concentration according to the work optimized by Yoon et al. 42 In contrast to other studies, 42,43 we have decided to keep a xed initial adsorbent concentration and to focus on the effect of pH on the adsorption process.In this study the pH conditions have a great impact not only on the stability of the suspension of NPs and their surface charges, but also on the type of speciation of phosphate involved in the adsorption process.Before starting the experiments, the pH of both the adsorbent solution and phosphate solution was adjusted to the desired value.Phosphate was added to NP solutions, followed by stirring at RT for 24 h.Samples were taken at different times and centrifuged (Eppendorf AG, Germany) at 13 000 rpm for 1 h.The phosphate adsorption efficiency was measured through UV-vis spectroscopy (UVIKON XL, Beun De Ronde) by using the ascorbic acid method. 44,45sorption and desorption cycles Fe 3 O 4 @PAHGu NPs were subjected to three adsorption and desorption cycles to test the reversibility of the binding process as well as the reusability of the coated NPs.In more detail, a solution containing 5 mg L À1 of NaH 2 PO 4 was added to 3 mL of a 0.5 g L À1 Fe 3 O 4 @PAHGu suspension at pH ¼ 5.The adsorption experiment was performed by stirring at RT for 24 h followed by a centrifugation step (1 h at 13 000 rpm).The desorption experiment was done by adding a 10 mM NaCl solution to the Fe 3 O 4 @PAHGu NPs that were obtained aer the centrifugation step of the rst adsorption.The solutions were stirred for 3 h at RT and the resulting solution was separated from the NPs by centrifugation (1 h at 13 000 rpm).Before the 2 nd and 3 rd adsorption cycle, Fe 3 O 4 @PAHGu NPs were washed twice with MilliQ water while stirring for 30 min each time aer which the water was removed by centrifugation (1 h at 13 000 rpm, each time).All phosphate-containing solutions were analysed through Ion Chromatography (930 Compact IC Flex, 150 mm A Supp 5 column, Metrohm).

Results and discussion
Given the importance of electrostatic interactions in the surface modication using polyelectrolytes, 33,46 we rst present and discuss the z-potential data of the PAH and PAH-Gu separately, and For the unmodied Fe 3 O 4 NPs the zeta potential changes from a positive (Fe-OH 2 + groups are in excess) to a negative (Fe-O À groups are in excess) sign around pH ¼ 7, reecting the PZC as has been reported in the literature. 47n contrast, PAH and PAH-Gu polyelectrolyte solutions remain positive over the whole investigated pH region.PAH-Gu shows a higher positive surface charge compared to the (unfunctionalized) PAH.This can be easily understood in terms of the respective pK a values of PAH and Gu, which are 8-9 for the primary amine of PAH 48,49 and 13 for the guanidinium group present in PAH-Gu. 50Furthermore, for PAH-Gu the z-potential data at pH < 6.5 show a plateau behavior, which is absent for PAH and the Fe 3 O 4 NPs in the studied pH window.This indicates that the overall surface-charge density of PAH-Gu at pH < 6 is constant.This difference may be associated with the differences in the PZC of the respective materials, including a shi of the apparent dissociation constant of PAH (pK a(app) ) due to local changes of the electrostatic environment 51 and, for PAH-Gu a saturation of chargeable groups under acidic pH conditions.
To conclude this part, the results show that within the pH window of $7 to $9.5 the unmodied Fe 3 O 4 NPs are negatively charged, while both PEs are positively charged.In addition, from the literature it is known that Fe 3 O 4 NPs are maximally covered by weak polyelectrolytes (like PAH) if the pH is similar to the polyelectrolyte pK a value. 52We have therefore chosen to perform our experiments at a pH of 9.5, the pK a value of PAH, for both PAH and PAH-Gu modications.

Characterization of coated Fe 3 O 4 NPs
The FTIR spectra of bare Fe 3 O 4 NPs, Fe 3 O 4 @PAH, and Fe 3 -O 4 @PAH-Gu as well as those of the pure PEs are shown in Fig. 2. The data show that the modied NPs are covered with PAH or PAH-Gu.In all cases a large contribution between 3404 cm À1 and 3017 cm À1 is observed, which can be associated with the O-H bond stretching.Its broadness originates from Hbridge formation with physically adsorbed water, which was used as a solvent and can be entrapped between the polymeric chains (see also TEM and TGA analysis; vide intra). 53The presence of iron oxide is conrmed by the observed stretching of Fe-O at 577 cm À1 in the cases of Fe 3 O 4 NPs (black, dashed), Fe 3 -O 4 @PAH (blue, dashed) and Fe 3 O 4 @PAH-Gu (red, dashed). 54he success of the PAH coating process becomes clear from the typical peaks at 2918 cm À1 and 2850 cm À1 that are associated with C-C stretching and two peaks at 1575 cm À1 and 1541 cm À1 of the C-N and N-H bending, which compare well with bands present in the FTIR spectrum of PAH (blue).Finally, the bands at 1604 cm À1 and 1506 cm À1 can be assigned to the bending vibration related to the amino group. 55Likewise, in agreement with the bare PAH-Gu spectrum (red), the coating of NPs with PAH-Gu is conrmed by the presence of two peaks at 2918 cm À1  and 2850 cm À1 for C-C stretching, a peak at 1631 cm À1 assigned to the stretching of C]N bond from the Gu group and a peak at 1537 cm À1 of the N-H bending. 38,56It should be mentioned that in both spectra of Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu, the positions of the characteristic peaks of primary amine and amide bonds shi to some extent with respect to the corresponding bare PEs.This can be explained by the interaction between iron ions and charged groups of PEs and the formation of amino complexes. 57PS was used to further map the surface chemistry of NPs before and aer modication (Table 1).The successful NP functionalization is evident from the N/Fe ratio that increases upon the preparation of the coating from 0 (bare Fe 3 O 4 NPs) to 0.30 and 0.60 for Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu, respectively.Moreover, with respect to the bare NPs, the C/Fe ratio is higher in the presence of the PEs and this can be associated with the alkyl polymer backbone and methylene groups in the side chains.Oxygen is measured in all samples, which can be related to OH groups present at the Fe 3 O 4 NP surface and the C]O in the Fe 3 O 4 @PAH-Gu system.
Carbon is detected in Fe 3 O 4 as well, and this can be related to hydrocarbon surface contamination oen observed on surfaces. 58While the C/Fe ratio of Fe 3 O 4 @PAH is higher than that of Fe 3 O 4 @PAH-Gu, the contribution of carbon contamination makes it hard to draw any conclusion on the degree of coverage based on C/Fe.An indication of the amount of PEs bound to the NP surface can be deduced from the N/Fe ratio.Taking into account a degree of Gu group substitution of 30% (see the chemical structure reported in Scheme 1), the calculated amount of N per repeating unit in PAH-Gu is 1.9 times higher than that for PAH (considering 0.3 Â 4(N) + 0.7 Â 1(N)).From XPS analysis a ratio of (0.60/0.30) ¼ 2 was observed, indicating that a similar amount of both polyelectrolytes is bound to the NPs.
Additional evidence of the changed surface chemistry of the NPs was obtained from TGA analysis (Fig. 3).Bare Fe 3 O 4 (line a) showed hardly any weight loss for the indicated temperature range (residual of 98%).This small weight reduction can be attributed to the loss of water physically adsorbed at the NP surface combined with the loss of condensed groups at temperatures higher than 100 C. 59 In contrast, two degradation steps clearly appear for Fe 3 O 4 @PAH (line b) and Fe 3 O 4 @PAH-Gu (line c).The rst step at 30-120 C refers to the loss of water.The presence of water is due to both physically adsorbed water at modied NP surfaces and the hydration shell of ions (ammonium and chlorine) of the polyelectrolyte chains, which is found to be almost the same for both systems (in accordance with the FTIR spectra).The second weight loss at 250-400 C can be related to the breakdown of the PEs.The residuals of Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu overall drop to 87% and 84%, respectively.The weight drop can be attributed to the bonded polyelectrolyte at the NP surface.The difference between the drop for Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu is due to the Gu modication, considering that the average mass per monomer unit is larger for PAH-Gu than that for PAH.
The morphology of NPs was examined with TEM; the images of Fe 3 O 4 NPs, Fe 3 O 4 @PAH, and Fe 3 O 4 @PAH-Gu are shown in Fig. 4. In the absence of a polymeric coating, Fe 3 O 4 NPs show a typical spherical shape. 60The same spherical shape can also be observed in images (b) and (c); in addition, a smooth and transparent layer is seen around the NPs, likely due to the presence of the polymeric coating (c). 61Table 2 lists the mean diameters of the NPs as obtained from TEM analysis.Fe 3 O 4 NPs were found to have a diameter of 8 AE 2 nm, conrming the specications given by the supplier.The diameters of Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu, including the additional smooth layer, are 11 AE 2 nm, indicating an adsorbed polyelectrolyte layer thickness of ca. 3 nm (TEM-based size distribution plots are presented in ESI Fig. S2 †). 62able 2 summarizes the size data of our investigated NPs as obtained from TEM and DLS (hydrodynamic diameters) and zeta potential measurements.At pH ¼ 9.5, the unmodi-ed and polyelectrolyte-modied NPs have hydrodynamic diameters much larger than the sizes of single particles observed by TEM.This is due to the agglomeration of these NPs in solution.This agglomeration is reduced for the NPs modied with a polyelectrolyte: 86 nm observed for the unmodied Fe 3 O 4 NPs, compared to 65 nm and 41 nm for the Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu, respectively.The difference is related to the colloidal stability, which is increased for polyelectrolyte-modied NPs, thus preventing aggregation. 29,30,58,63,64We also observed a stable suspension for both PE-modied NPs, while the unmodied Fe 3 O 4 NPs precipitated aer 24 h at pH ¼ 9.5 (Scheme 1, bottom le and ESI Fig. S3 and S4 †).The stability of the NP suspension at pH ¼ 9.5 as observed from DLS was conrmed with z-potential, which changes sign upon modication to +26 mV and +32 mV for the PAH and PAH-Gu coatings, respectively.The positive z-potential values strongly conrm the presence of polycations at the NP surface. 61oreover, it should be noted that the magnitude of the surface potential reects the level of electrostatic repulsion between NPs.A higher zeta potential gives more repulsion and therefore a more stable suspension.From these zeta potential measurements it is now clearly understood why the Fe 3 O 4 NPs start to agglomerate, while the PE-modied NPs are still stable.From the results obtained, it is evident that the addition of a Gu moiety altered the Fe 3 O 4 properties; this is in terms of not only reversing the surface charge to a positive value (as is the case for Gu-free PAH), but also increasing the absolute charge density, leading to an increased colloidal stability.Again, this can be explained by the differences in the PZC between the amino-PAH and Gu moiety. 65 present, while at pH ¼ 10 mostly HPO 4 2À can be expected. 22The adsorption experiments were performed at a xed concentration of 0.5 g (modied) NP per L and 2 mg NaH 2 PO 4 per L; thus there is always an excess of adsorbent.Fig. 5 shows the results of the phosphate adsorption as a function of pH for Fe 3 O 4 NPs, Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu aer equilibration for 24 h at RT as determined by the ascorbic acid/UV method. 45t pH ¼ 5 all NPs show a similar amount of phosphate adsorbed.Under these conditions, the phosphate is predominantly present as the mono-anion (pK a1 ¼ 2.1 and pK a2 ¼ 7.2) and the Fe 3 O 4 NPs are below their PZC and therefore will have a net positive charge.For the PAH and PAH-Gu modied Fe 3 O 4 NPs also the net surface charge is positive.The phosphate monoanion will therefore bind to the unmodied Fe 3 O 4 NPs, as reported in the literature. 19Since there is hardly any extra effect of the PAH and PAH-Gu modications on the adsorbed phosphate amount it is suggested that the Fe 3 O 4 NP surface determines the adsorption under these conditions.Despite the positive charges at the Fe 3 O 4 NP surface, the stability of the PE coating under this pH condition can be related to the presence of neutral amino groups in the PAH and PAH-Gu chains.It is likely that both positive charges and neutral hydroxyl groups present on the Fe 3 O 4 NP surface interact with the unprotonated amino groups of PEs.
Increasing the pH from 5 to 8 and 10 shows a reduction of phosphate adsorption by the unmodied Fe 3 O 4 NPs of 46% and  77%, respectively.At these pH values, the surface charge has turned to a negative value and the adsorption of phosphate monoanions or dianions is suppressed by electrostatic repulsion.Yet, at pH ¼ 10 the phosphate adsorption is not reduced to 0; instead, it is still 0.85 mg g À1 .Thus, the adsorption of phosphate onto iron-oxide surfaces occurs both by electrostatic interactions, absent at pH ¼ 10, and by a chemisorption process. 66,67The latter involves the formation of Fe-O-P bonds through a ligand exchange reaction between OH groups at the NP surface and phosphate oxygen.This may explain the P adsorption detected at pH ¼ 10.
A very clear difference is observed for pH ¼ 8 and pH ¼ 10, if PAH or PAH-Gu is present.The amount of adsorbed phosphate is now higher than that observed for unmodied Fe 3 O 4 NPs and more or less similar to the adsorbed amount observed at pH ¼ 5 for the three investigated NPs.Clearly, the reduced affinity of the (unmodied) Fe 3 O 4 NP surface at pH ¼ 8 is compensated nearly completely by the PAH and PAH-Gu modications.For pH ¼ 10, it is seen that the phosphate adsorption for PAH is decreased compared to that of the PAH-Gu modied surface.For PAH-Gu still a phosphate adsorption of 3.67 mg g À1 is observed.This difference nicely reects the difference of the pK a values of PAH (8-9) and PAH-Gu (Gu groups pK a ¼ 13) due to which the latter has a higher positive charge density at pH ¼ 10.
In addition, the increased stability of the colloidal suspension may contribute to the uptake of phosphate, because a higher contact area is available compared to the aggregated state.Increased colloidal stability is supported by z-potential measurements: at pH ¼ 10 a zeta potential of +21.1 mV and +3.8 mV is found for Fe 3 O 4 @PAH-Gu and Fe 3 O 4 @PAH, respectively.As mentioned previously, the adsorption of phosphate slightly decreases upon increasing pH.Under alkaline conditions, OH À groups are abundant and they might compete with phosphate in the adsorption process. 19,28fect of contact time and adsorption kinetics Phosphate adsorption was monitored as the decrease of the phosphate concentration over time at pH values of 5, 8 and 10 (Fig. 6).At pH ¼ 5 all NPs show a very fast adsorption behavior.Equilibrium was reached within 5 min.Due to our experimental setup we are not able to accurately monitor the adsorption increase within that time frame.However, it is clear that at pH ¼ 8 and pH ¼ 10 the adsorption process is slower, making monitoring of the adsorption increase possible.Equilibrium is now obtained within 1 h.This is similar to observations made by others. 29,68he monitored increase of phosphate adsorption as a function of time can be nicely tted with a pseudo-second-order kinetic equation. 69,70 where q e is the amount of phosphate adsorbed at the equilibrium, q t is the phosphate adsorbed during the time t and k 2 is the pseudo-second-order rate constant.The equation describes the increased amount of adsorbed phosphate over time as a function of the difference between q e and q t .Although other kinetic models are reported in the literature to describe adsorption processes (i.e., pseudo-rst-order, Elovich), the Measurements were done in triplicate and all errors were found to be lower than 0.05%.
pseudo-second-order is widely recognized as the best model particularly at low initial solution concentration. 71The t is shown for the linearized form of eqn (1), which is given by eqn (2) (tting plots are reported in Fig. S5, † while the non-linear curve tting parameters are listed in Table S1 †): In Table 3 the results of the tting q e and k 2 are compiled together with the calculated value of the initial rate at t / 0 (h in mg g À1 min À1 ), and the coefficient of determination (R 2 ), reecting the quality of the t.
As said, under conditions of pH ¼ 5 the process is too fast for monitoring adsorption increase data and therefore we only report here experimental values of q e .Under conditions of pH ¼ 8 and 10 the monitored data of increased adsorption tted very well with the second-order kinetic equation as deduced from the obtained coefficients of determination close to unity.The observed second-order behavior is a net result of the combination of adsorption and desorption processes occurring simultaneously. 71While it is realized that the pseudo-secondorder kinetics is oen ascribed to a double-site interaction, 42,70 we point to the derivation of the pseudo-second-order rate equation from the Langmuir kinetics as described by Liu and Shen. 71Double-site adsorption would be a correct physical interpretation, only if the binding sites involved can move independently over the surface and need to be close in order to bind one phosphate.However, the work of Liu and Shen 71 demonstrates that the combination of the simultaneous adsorption and desorption processes also leads to apparent second-order kinetics when the total amount of binding sites per unit of volume is larger than both the initial concentration of the adsorbate and the inverse of the equilibrium binding constant.The fact that we observe second-order kinetics implies that these conditions are met.
No physical meaning can be attributed to k 2 , 72 but the values for the initial adsorption rate (h) and the amount of adsorbed phosphate at equilibrium (q e ) can be interpreted.At both pH ¼ 8 and pH ¼ 10, h increases from Fe 3 O 4 NPs to Fe 3 O 4 @PAH to Fe 3 O 4 @PAH-Gu.At pH ¼ 10, q e also increases in this order.In contrast, at pH ¼ 8 for Fe 3 O 4 @PAH and Fe 3 O 4 @PAH-Gu a similar order of q e is observed, which is higher than that of Fe 3 O 4 NPs.It is suggested therefore that at pH ¼ 8 the adsorption capacity of the two investigated polyelectrolytes is similar.The difference between the two polyelectrolytes becomes visible at pH ¼ 10, in favor of Fe 3 O 4 @PAH-Gu, showing a pH-independent value of q e .This is likely due to the differences in the pK a of the PAH and PAH-Gu PEs; the Gu moieties are still protonated at pH ¼ 10, while for PAH the degree of protonation is reduced compared to the situation at pH ¼ 8. From the results shown in Table 3, it is also clearly seen that at pH ¼ 8 and pH ¼ 10 the phosphate adsorption is dictated by the present PEs and that the dominant role observed for Fe 3 O 4 at pH ¼ 5 is now tempered.An additional difference between Fe 3 O 4 @PAH and Fe 3 O 4 @-PAH-Gu (not shown here) is the selectivity for phosphate binding for the Gu containing polyelectrolytes, which we have shown in our previous study. 38

Reversibility of phosphate binding
The reversibility of the adsorption process is highly relevant when it comes to practical applications.Initially we run desorption experiments under high alkaline conditions 43 (pH ¼ 12.9) to weaken the electrostatic interaction between PAH-Gu and phosphate.While a high phosphate desorption was obtained this way (>80%), the NPs were found to agglomerate and it was difficult to get them re-dispersed.Hence, the second adsorption cycle was unsuccessful and the NPs were not reusable, likely caused by a partial removal of the PAH-Gu coating, which acts as a coagulant for Fe 3 O 4 NPs. 54However, phosphate could be removed successfully from Fe 3 O 4 @PAH-Gu NPs by a regeneration process using 10 mM of NaCl solution through an anion-exchange mechanism.Phosphate adsorption and desorption were monitored over three cycles (Fig. 7) and high levels of phosphate recovery were reached in good agreement with previous work reported in the literature. 26,42Aer the rst cycle, the adsorption of phosphate was decreased in the next cycle by almost 20%, which might be explained by phosphate being irreversibly bound to (and/or physically entrapped in) the NPs and PAH-Gu network or to a decrease of the available active surface caused by a partial NP aggregation aer the centrifugation steps.
Table 3 Kinetic model parameters obtained from pseudo-second-order model fitting to experimental time-dependent adsorption data for phosphate on Fe 3 O 4 , Fe 3 O 4 @PAH, and Fe 3 O 4 @PAH-Gu at pH ¼ 5, 8 and 10.For completeness, the q e values experimentally determined at 24 h (q e exp) are included pH 5 pH 8 pH 10 q e exp mg g À1 q e exp mg g À1 q e mg g À1 k 2 mg g À1 min À1 h mg g À1 min À1 R 2 q e exp mg g À1 q e mg g À1 The initial rate of phosphate adsorption increased from 2.1 to 29 mg g À1 min À1 for PAH-Gu coated Fe 3 O 4 NPs upon switching the pH from 10 to 8. The observed second-order adsorption kinetics can be explained as the net result of simultaneous adsorption and desorption processes at the NP surface.At the same time, the colloidal stability was enhanced upon coating the NPs with polyelectrolytes.Finally, the reversibility of phosphate binding to the novel Fe 3 O 4 @PAH-Gu NPs was studied over three cycles of adsorption and desorption, showing the reusability of the NPs.While already most (>80%) of the bound phosphate could be released again, we believe that the efficiency can be further improved by additional advanced surface modication strategies, e.g. by covalently binding the coating to the NPs and capping the remaining surface hydroxyl groups.
Fig.1shows the z-potential of bare Fe 3 O 4 NPs as well as of PAH and PAH-Gu in aqueous solutions as a function of the solution pH.It is observed that for all cases the zeta potential becomes less positive with increasing pH value.For the unmodied Fe 3 O 4 NPs the zeta potential changes from a positive (Fe-OH 2 + groups are in excess) to a negative (Fe-

Fig. 1
Fig. 1 z-Potential as a function of solution pH for (a) an aqueous suspension of unmodified Fe 3 O 4 NPs (0.5 g L À1 , black squares), (b) an aqueous solution of 0.5 g PAH per L (blue circles), and (c) an aqueous solution of 0.5 g PAH-Gu per L (red triangles).

4 À
Images of the NP suspension at different pH values aer 24 h and Fe 3 O 4 @PAH-Gu aer 1 week are shown in Fig. S2 and S3 in the ESI.† Phosphate adsorption: effect of pH In order to map the pH-dependency of phosphate adsorption at our (modied) NPs, three pH values were chosen for the adsorption experiments: pH ¼ 5, pH ¼ 8 and pH ¼ 10.Within the pH window from 5 to 10 the degree of dissociation of phosphoric acid decreases accordingly, thus at pH ¼ 5 H 2 PO is predominant, at pH ¼ 8 H 2 PO 4 À and HPO 4 2À are equally

Fig. 5
Fig. 5 Amount of phosphate adsorbed (mg P g NPs À1 ) after equilibration for 24 h at RT for (a) Fe 3 O 4 NPs (black), (b) Fe 3 O 4 @PAH (blue), and (c) Fe 3 O 4 @PAH-Gu NPs (red).The dashed lines serve as a guide to the eye.Measurements were performed in triplicate and all errors were found to be <0.05%.

Fig. 7
Fig.7Recovery of phosphate for Fe 3 O 4 @PAH-Gu over three cycles; empty columns give the adsorption data after 24 h, while the filled columns show the desorption results.NPs were regenerated with a 10 mM NaCl solution at RT and pH ¼ 5, the adsorbent dosage was 0.5 g L À1 and the initial phosphate concentration was 5 mg L À1 .

Table 1
XPS elemental ratios of bare and polyelectrolyte-modified NPs This journal is © The Royal Society of Chemistry 2017ConclusionsWhile several nanomaterials have been investigated for the removal of phosphate from aqueous (wastewater) streams, it remains a challenge to develop new systems operable under alkaline conditions.This study shows the results of a simple surface modication method applied to commercially available Fe 3 O 4 NPs by using a polyelectrolyte functionalized with guanidinium groups for phosphate anion binding.The surface modication was conrmed by thermal, morphological and surface analysis measurements (FTIR, XPS and z-potential analysis).The PAH-Gu modied Fe 3 O 4 NPs showed good phosphate adsorption (3.7 mg g À1 ) up to pH ¼ 10, where the phosphate adsorption ability of the PAH-modied Fe 3 O 4 (2.3 mg g À1 ) and unmodied Fe 3 O 4 (0.82 mg g À1 ) is reduced.