Determination of interactions of ferrihydrite-humic acid-Pb (II) system

Ferrihydrite often precipitates with humic acid in natural ways, affecting the fate of lead ions, the stabilization of humic acid, and the aging process of ferrihydrite. A series of 2-line ferrihydrite-humic (Fh-HA) acid with varying C loadings has been prepared, the morphology and surface properties of Fh-HA organo-minerals have been characterized, and the adsorption property of Pb ions onto Fh-HA has been studied. The results indicated that a strong interference of HA to ferrihydrite 2-line Fh dominated mineral phase in all samples, but with increasing C/Fe molar ratios, the crystallinity gradually weakened, particles became smaller, and SSA decreased significantly. The data of Mössbauer spectra confirmed C loadings changed the unit structure of ferrihydrite. Fh-HA performed good adsorption properties to Pb (II): high efficiency and big capacity, especially Fh-HA_2.0. pH had great effect on Pb (II) sorption, the pH change affects not only the amounts of competitive ions in solutions, but also the dissociation and protonation of functional groups on the surface of Fh-HA. Sorption kinetics can be well modeled by a pseudo-second-order model, and the process was controlled by film and intraparticle adsorption simultaneously. The adsorption isotherms can be well described by Freundlich isotherm model. The detailed determination results of Fe 2p, O 1 s, and Pb 4f spectra before and after lead adsorption showed mononuclear bidentate or binuclear bidentate ligands occurring on Fh-HA surface, forming stable inner-sphere complex. By comparison of Mössbauer spectra and TEM images, with aging time, a slower evolution of iron oxide/oxyhydroxide phases in Fh-HA-Pb system happened compared to pure ferrihydrite. Ferrihydrite has transformed to a combination of ferrihydrite, goethite, and hematite phases. In this study, the determination of C-Fe interaction, Pb fate influenced by Fh-HA, and transformation of ferrihydrite would have a great implication to application of Fh-HA precipitates in remediation for surface water or groundwater polluted by heavy metals.


Introduction
is present at elevated concentrations in air, soil, and aquatic environment, and the emission is from a variety of anthropogenic sources, including municipal and industrial waste, metal mining processing, and agricultural activities (Pan and Wang 2012). Various lead origins make it the most often encountered metals to groundwater and soils (Mohamed et al. 2019). The lead concentration in some soil samples in China was up to 1386 mg/kg (Xueqiu et al. 2019). As a non-degradable pollutant, Pb has a high mobility, bioavailability, and toxicity (Turner et al. 2019). Accumulating exposure of lead poses existing and potential risks and threats to human health and ecological safety , and the toxicity ranks very high (the second) in the official report (http:// www. arsdr. cdc. gov/ spl).
Ferrihydrite (Fh), as poorly ordered Fe minerals, distributed in soils, sediments, and some aquatic environments ubiquitously (Rout et al. 2012), is the precursor of some crystalline iron oxides. Naturally, due to cooperation effect of a large specific area and a high reactivity, Fh can adsorb metal ions and organic molecules effectively (Cornell and Schwertmann 2003). Freshly prepared Fh is mostly efficient adsorbent known for Pb, As, Cu, and Zn sequestration (Zhu et al. 2010), affecting their fate and mobility in the aqueous phase. It plays an important role in wastewater treatment and remediation activities (Kragovic et al. 2017;Weiqiang et al. 2019).
The widely distributed natural organic matter (NOM) always co-exists with iron (hydr)oxides by complexation or coprecipitation. Humic acid, as a typical representative for NOM, contains carboxyl, aromatic, phenolic functional groups, which are often closely associated with minerals through hydroxyl groups, forming organo-mineral composites (Fortin and Langley 2005). C loadings can change surface and structure properties of iron (hydr)oxides; for ferrihydrite, a new negative charge surface and more available sites formed by coprecipitation with humic acid, thereby affects the adsorption properties of Pb (II) onto ferrihydrite (Tiberg and Gustafsson 2016). HA has been found to inhibit and retard crystallization and transformation processes (Cornell and Schwertmann 1979;Gustafsson et al. 2007), altering geochemical reactivity of ferrihydrite. The interaction between humic acid and ferrihydrite is mainly through electrostatic attraction, ion exchange, hydrogen bonding, and cation bridging. The interaction changes the way that the ferrihydrite forms and geochemical behavior. The mobility of ferrihydrite is affected by humic acid concentration. Low concentration will inhibit the fluidity of ferrihydrite, while high concentration will promote (Liao et al. 2016). This is because excess humic acid molecules enhance electrostatic and spatial stability. In heavy metal remediation, the Fh-HA complex by coprecipitation has stronger adsorption-desorption to metal ions (Du et al. 2018;Xue et al. 2019), producing the effect of 1 + 1 > 2. It is reported that in coprecipitation, amount of C loading can significantly alter XRD patterns and Mössbauer spectrum of ferrihydrite, and the maximum achieved C loading is limited; 170 mg C and 318 mg C per g ferrihydrite are achieved respectively in some previous researches (Eusterhues et al. 2008;Kaiser et al. 2007). Based on this, it is necessary to set proper C/ Fe ratio range of the initial solution in synthesis of Fh-HA coprecipitates experiments. Fe (O, OH) 6 octahedron is characteristic structural unit in all Fe (hydr)oxides, and the chains formed by the unit are cross-linked at corners of octahedron to neighbor chains. However, the specific Fe arrangement in ferrihydrite structure is still under discussion. Furrer and Stumm reported that when the humic acid adsorbs onto the iron hydroxides, the bidentate ligand with two oxygen donors of the HA reacts with Fe hydroxides under acidic condition (Gerhard Furrer & Werner Stumm, 1986). In recent studies, researchers demonstrated how reaction occurs in Pb and synthetic composites, and the synthetic composites exhibited an excellent performance (Xinyi et al. 2021;Zhao et al. 2021;Wang et al. 2017).
Among the existing researches (Ding et al. 2018;Lu et al. 2019;Lei et al. 2018), it is noted that the interaction of Fh-HA-Pb system and the effect of associated HA and Pb on geochemical reactivity of ferrihydrite on the molecular scale has received little attention. To facilitate the dearth of regarding knowledge, the possible change of Fh-HA coprecipitates with varied C loadings (C to Fe molar ratio is 0.5, 1.0, 1.5, 2.0, 2.5) has been examined; we elucidate physical and structural properties of Fh-HA by means of X-ray diffraction (XRD), transmission electron microscope (TEM), and BET and Mössbuaer spectra. Batch experiments have been conducted to analyze Pb sorption behavior onto Fh-HA. Thermal and kinetic models were constructed based on detailed reaction data to describe Pb (II) transport in water-mineral interface. The molecular structure, bonding characteristic, and compositions of Pb (II) complexes, and the acid/base properties has also been determined to characterize the interactions of Fh-HA-Pb ternary system combined with a series of techniques such as XPS, Mössbauer spectroscopy. The findings of this work could provide a better understanding of the Pb mobility and fate affected by Fh-HA in contaminated environment, also facilitate our knowledge about the way ferrihydrite transformed when humic acid introduced and Pb adsorbed.

Materials and methods
All reagents and chemicals were of analytical grade, all solutions were prepared using deionized water (DIW) (≥ 18 MΩ cm), and CO 2 was rigorously excluded to prevent the formation of Pb (II)-carbonate aqueous complexes or precipitates. Stock solutions of FeCl 3 and Pb(NO 3 ) 2 were made by dissolving metal salts in DIW. HA was purchased from XIYA (1415-93-6. Shandong, China), and was dissolved with 0.4 M NaOH. The total C content in HA was measured at 61.98wt%. Glassware used for all synthesis steps was rinsed in DIW, soaked in 0.1 M HNO 3 for at least 24 h, and triple-rinsed in DIW.

Synthesis of Fh-HA coprecipitates
Pure two-line ferrihydrite (Fh) and five Fh-HA coprecipitates (0.5, 1.0, 1.5, 2.0, and 2.5) were prepared according to the procedures adapted from Schwertmann and Cornell (Schwertmann and Cornell 2000). In brief, Fh was synthesized by hydrolysis of Fe 3+ as FeCl 3 ·6H 2 O in ultrapure water, rapidly adjust solution to pH ~ 7.5 using freshly prepared 1 M NaOH, under vigorous stirring for 2 h, subsequently was washed, centrifuged, and freeze-dried. For Fh-HA coprecipitates, firstly, humic acid soluble in sodium hydroxide was added and mixed with a previously prepared Fe 3+ salt solution, containing elevated initial C concentrations, then hydrolyzed and adjusted to pH ~ 7.5, subsequently operated in the same way as preparation procedure of ferrihydrite. The five Fh-HA coprecipitates are termed hereafter as C/Fe_0.5, C/Fe_1.0, C/Fe_1.5, C/Fe_2.0, and C/Fe_2.5 accordingly. All freshly prepared Fh and Fh-HA were used as a standard for measurement and in all lead adsorption experiments. Surface chemistry was characterized by XRD, TEM, and BET.

Lead adsorption experimental designs
All Pb (II) adsorption experiments were processed at room temperature, and 0.01 M NaNO 3 was used as the background electrolyte. The affected factors such as pH, ionic strength, initial Pb (II) concentrations, and adsorbent dosage were introduced to the system separately to shed light on adsorption occurring conditions. For each factor, reasonable value range and gradient were designed, to determine the optimal adsorption aqueous environment. All samples were in triplicate. For adsorption isotherms, the adsorption contained 6 g/L adsorbent and 0-16 mmol/L Pb (II). For adsorption-desorption kinetics, common functions of combined effected factors were taken into account. In the duration of this experiment, sampling time interval was set up, and the supernatant was collected during adsorption reaction at 15 min, 30 min, 60 min,120 min, 240 min, and 1440 min. The same sampling time interval for desorption and the desorption was performed for two cycles. All finished processing suspensions were immediately allowed to equilibrate for 24 h at a shaker. To remove adsorbent particles, the suspensions were centrifuged, and the obtained supernatants were purified through 0.22-um membrane filters. The Pb (II) concentration in supernatant was determined using an atomic absorption spectroscopy after appropriate dilution (ZEEnitt700P Analytik Jena AG., Germany); the basic instrument settings to measure Pb (II) is as follows: Characteristic spectral line is 283.3 nm, lamp current is 8.0 mA, slit width is 0.2 nm, and the current of C 2 H 2 -air is 65 L/h. All residual dense paste from the precipitated part of the reaction solution after centrifugation was for analysis by XPS, Mössbauer spectroscopy, and TEM equipped with energy dispersive X-ray spectroscopy (EDX).

XPS and Mössbauer spectroscopy analysis
X-ray photoelectron spectroscopy (XPS) is used to identify adsorbed species on the surfaces of the Fh-HA. The XPS spectra of the precipitates that reacted and unreacted were determined by a Kratos Axis spectrometer (Kratos Analytical, Manchester, UK), with application of monochromatic Al Kα-radiation with an excitation energy of 1486.7 eV. Survey spectra were acquired using a pass energy of 160 eV and a step size of 1 eV. A high-resolution scan for O, C, Fe, and Pb element was acquired with a pass energy of 160 eV and a step size of 0.05 eV. All binding energies (BE) were calibrated by reference to the C1s peak at 284.8 eV (Chen and Wang 2007). The XPS spectra were fitted and analyzed by means of curve fitting program Casaxps. Mössbauer spectra were measured in transmission geometry employing a source of 57 Co in Rh and a sinusoidal velocity waveform. A Kr proportional counter equipped with single channel analyzer windows, setting on both the 14.4-keV photo peak and the escape peak, was used to detect the 14.4 keV γ-rays. Mössbauer spectrometer calibration was performed at room temperature using a standard SNP. For the given isomer shifts, to eliminate contributions from the second-order Doppler shifts, which are always in respect of the 57 Co-Rh source keeping the same temperature as the absorber. The raw data were folded and the geometry effect eliminated, and then least-square with Gaussian distributions of hyperfine parameters. Further details of the distributions will be discussed in results and discussion section.

Surface and morphology
Ferrihydrite displays nanometer sized particles in width and several microns in length. The specific surface area (SSA) of ~ 300 m 2 /g determined for the 2-line ferrihydrite is supported by published data (Leone et al. 2006). All Fh-HA precipitates had much smaller SSA and porosity compared ferrihydrite (Table 1). In practice, the combined effect of charge stabilization and steric stabilization affect the final average sizes of Fh-HA colloidal particles in soils and aqueous environment. Fh and several Fe precipitates materials have similar N 2 adsorption edges. N 2 adsorption capacity under low pressure is high, which agrees well with typical IV adsorption patterns, it is an indicator that Fh and all Fh-HA samples are porous materials with mainly mesopores, small amount of micropores, (Fig. 1), and the result is consistent with the pore size distribution. However, with increasing C amount, the N 2 -detectable mineral pore volume of Fe precipitates declined by up to 50% compared to pure Fh.
The XRD of the synthetic ferrihydrite at ~ 0.26 nm and ~ 0.15 nm shows a characteristic peak of 2-L Fh (Fig. 2). Their diffuse shoulders are present in smaller peaks at 0.22 nm, 0.20 nm, 0.17 nm, and 0.16 nm, which accords to standard JCPDS card no. 29-0712. For coprecipitates, differences in XRD pattern get bigger with an increasing C/Fe molar ratio; two main peaks paralleled to Fh broaden and some tiny peaks at 0.22 nm, 0.20 nm, 0.17 nm, and 0.16 nm weaken until finally disappear. The change implied a strong interaction between Fh and HA, resulted in increasing disorder in the anionic layers, indicative of formation of crossed small chains of Fe octahedra units in the precipitation process (Cornell and Schwertmann 2003). The degree of structural deformation TEM images and corresponding selected area electron diffraction (SAED) patterns of the six samples are given in Fig. 3. Fh forms aggregates with no surface roughness, a spherical geometry with hexgonal edges and consists of particles ~ 3-5 nm in diameter. This coincides with the measured SSA of ~ 310 m 2 g −1 (Table 1). It has been reported that the developed hexagonally shaped particles are indictor of Fh with higher structural order (Eusterhues et al. 2011). While smaller particles formed in the presence of HA in spite of its smaller SSA, it can be explained by the formation of denser aggregates by the associated HA resulted in a reduced accessibility for N 2 ; thus, less surface was detected. The SAED pattern of all samples is indexed to the (110), (112), and (115) planes of ferrihydrite (Mikutta et al. 2008). With increasing C loading, the corresponding SAED shows less distinct diffused rings. The images combined with XRD analysis showed the shape of sample particle either in hexagonal (Fh) or spherical shape (Fh-HA) with a size of 2-4 nm, more C Fig. 4 Mössbuaer spectra of Fh and Fh-HA coprecipitates taken at room temperature loadings, less ordered. In summary, our analyses confirmed that 2-line ferrihydrite was prominent mineral phase in coprecipitates, and C loadings affected and changed the surface properties, phase as well as the morphology.

Mössbauer spectra analysis
All spectra recorded at ambient temperature consist of a single, broadened electric quadrupole, possessing isomer shift (IS) of 0.36 mm s −1 (Fig. 4). The identical mean shift value for all samples is in accordance with the exclusive presence of high-spin Fe 3+ in an octahedral oxygen environment (Janney et al. 2000), considering trivalent Fe is in range of 0.24-0.54 mm s −1 (Bancroft 1974;Cardile 1988). Quadrupole splitting (Qs) was well fitted by an asymmetrical Gaussian distribution (Table 2). Increasing Qs value with increasing C/Fe implied affected spatial coordination of center Fe 3+ and the octahedral structure around the center Fe atom increasingly distorted. Some research indicated that at 4.2 K the Mössbauer spectra showed a broaden sextet, and with C/Fe ratios increased, the mean magnetic hyperfine field decreased; this was ascribed to decreasing particle size and crystallinity. Mössbauer spectrum confirming the XRD results, both showed that the Fe compounds in all prepared samples was single ferrihydrite phase, without presence of goethite or hematite derivatives. C loading affected not only the surface properties and morphology of ferrihydrite, but also the microscopic geometric structure centered on iron atoms, more C loaded, stronger effect generated.

Isotherms
Uptake data of Pb (II) adsorbed to coprecipitates isotherms at various pH are illustrated in Fig. 5. The Pb adsorption capacity increases evidently with the pH increased, particularly at pH 8, suggesting alkaline environment is favorable for Pb adsorption. The analysis of adsorption process applied Langmuir and Freundlich isotherm models, and all parameters of both models listed in Table 3. The Langmuir isotherm is given as: where q m (mmol g −1 ) represents the maximum adsorption capacity; q e (mmol g −1 ) is the amount of adsorbed Pb per g of Fh-HA at equilibrium; and K L is the Langmuir constant.
The Freundlich isotherm is given as: where K F and n are related to the adsorption capacity and the adsorption intensity, respectively. One can see that Freundlich yields a better fitting than the Langmuir model at pH 4 and pH 6 by comparing R 2 values, while Langmuir model is better at pH 8, the best R 2 for Langmuir isotherm and Freundlich isotherm is 0.962 and 0.965, respectively, at investigated pHs. The change of pH not only affects the amounts of competitive ions in solutions, but also has a significant effect on the dissociation and protonation of functional groups on the surface of Fh-HA. According to Langmuir fitting, the adsorption capacity, q m (mmol/g Fe), follows the order: pH8 > pH6 > pH4. The difference was normalized to their varying active sites density on surface, at higher pH, more proportion of carboxylic and phenolic groups in humic acid were dissociated; thus, more sites are available to complex Pb ions. Moreover, the higher K L -value and n-value in isotherm fittings at pH 8 indicating Fh-HA precipitates has stronger affinity for Pb (II) ions at (1) q e = K L q m C e 1 + K L C e (2) q e = K F C 1∕n e higher pH, and the result coincides with that obtained in desorption experiment, in which Pb (II) is harder to desorb from Fh-HA at higher pH. In summary, the isotherm results revealed that Pb adsorption occurs on heterogenous sites of Fh-HA, the acidic/base state has great effect on not only available sites, also the affinity to Pb ions.
The adsorption isotherms of Pb on the Fh-HA at 298 K, 303 K, and 308 K were investigated. The results (Fig. 6) showed that the adsorption capacity decreased with the increase of temperature indicating that the adsorption is an exothermic process.

Kinetics
It has been depicted in Fig. 7, three different kinetic models have been used to evaluate the adsorption kinetics of Pb (II) on Fh-HA precipitates. The pseudo-first-order, pseudo-second-order, and intraparticle diffusion formulas are given as (3), (4) and (5), respectively where q t (mg L −1 ) represents adsorption capacity at time t and k 1 (h −1 ) is the rate constant of the adsorption type.
where k 2 (L mg −1 g-1 ) represents the rate constant of the pseudo-second-order adsorption.
where k i (mg L −1 h −1/2 ) represents the rate constant of intraparticle diffusion, C (mg L −1 ) is a constant.
The tested parameters can be seen from Table 4, and a better R 2 was obtained for the pseudo-second-order kinetic model compared to the other two models. This means that Pb (II) adsorption properties onto Fh-HA are more in line with the pseudo-second-order rate expression, which indicates the ions concentration at the adsorbent surface (5) q t = k i t 0.5 + C  determines the Pb (II) adsorption rate. Obviously, during the whole adsorption process, two phases involved: for fast and slow stages (0-2 and 2-24 h, respectively). The two-stage plot suggests Pb adsorption process proceeds by surface adsorption and intraparticle adsorption (Xiu et al. 2018a, b), a boundary layer effect for the initial stage while pore diffusion for the second stage (Vadivelan and Vasanth Kumar 2005). The actual rate-controlling step involved in the adsorption can be determined by the calculation of adsorption dynamics, the following equation was employed, where F is the fractional attainment of equilibrium at time t and Bt is a calculated mathematical function of F derived from Reichenberg's table (Reichenberg D, 1953). Bt can be given as: B t vs t as proposed by Eq. (7) and fitted and plotted in Fig. 8. The linearity can be used to distinguish which mechanism predominates, film diffusion or intraparticle diffusion, and to estimate which factor controls the adsorption kinetics. Based on the point proposed by Wang et al. (2006), the intercept (C) was not zero and the plot is not a straight line passing through origin, which signify the film diffusion controlled the adsorption rate in whole process (Baskaralingam et al. 2006;Gupta and Bhattacharyya 2006;Wang et al. 2006). Based on the B t values, two plotted portions implied different mechanisms, the R 2 value for 2-24 h is higher than that for the 0-2 h portion, suggesting that the intraparticle diffusion dominated when the adsorption time becomes longer (Chen and Wang 2007).

Desorption
As shown in Fig. 9, in the desorption stage, the Pb (II) amount desorbed increased in sequence of the amount of Pb (II) retention in Fh-HAs, as expected, but the differences among adsorption amount were not dramatic. As expected, the Pb (II) desorption decreased as pH increased. However, compared with adsorption stage, pH had relatively small effect on the desorption stage because in re-adsorption reactions, strong coordination influenced much in the overall desorption process. Quick and much Pb (II) adsorption but slow and small desorption indicate a easily accessible sites with strong affinity for Pb ions on Fh-HAs. When initial Pb (II) concentrations and pH decreased, the desorption effect was more obvious; this can be explained by the significant contribution of medium and strong sites to Pb (II) (Tian et al. 2017).

Mechanism of Pb complex formed and phase evolution of ferrihydrite
The interaction mechanism, types of complexes between Fh-HA and Pb (II) were determined by XPS. As shown in Fig. 10a, survey spectra indicated as expected the presence of Fe, O, adventitious carbon, and Pb (Fig. 10a). The Pb 4f 7/2-5/2 core-level spectra acquired after Pb adsorbed on Fh-HAs were presented in Fig. 10d. Obviously, spectra of the Pb 4f had two bands, both closely coincide with standard  spectra for α-Pb, the Pb 4f 7/2-5/2 core-level spectra have high coincidence with the regular double 4f 7/2-5/2 line-shape of lead-oxygen compounds and Pb-metal bond (D.A. Zatsepin et al. 2017), indicating that the Pb ions was embedded into the Fh-HAs by bonding with oxygen in functional groups on Fh-HA. For Fe 2p XPS spectra, Fe (III) species in iron (oxyhydr)oxides was confirmed for it typical spectra mode, a maximum at ~ 711.2 eV for 2p3/2 core level and a broad satellite peak at ~ 719 eV were observed (Fig. 10c) (Grosvenor et al. 2004). The careful comparison of the position of the core level peak seemingly indicates a very slight shift toward higher binding energy after Pb ions adsorbed. Thus, the assumption that the formation of Fe-O-Pb binding according to spectra of Fe 2p and O1s is clearly confirmed here.
Through XPS peak separation and simulation, the main O 1 s XPS spectra of the Fh-HA were divided into three subpeaks (Fig. 11), every subpeak is for a component: ~ 529.9 eV is for oxygen in the crystal lattice (Fe-O-Fe), ~ 531.5 eV is for oxygen forming bonds as Fe-O-H or Fe-O-Pb or simply oxygen in the HA, and ~ 533.2 eV is for oxygen in carboxyl and phenolic groups in the HA (Tan X L et al., 2008). The component at ~ 531.5 eV was calculated and increased ~ 29% after Pb adsorption, while the components at ~ 531.5 eV and ~ 533.2 eV decreased. The change in the peak intensity and area is contributed by the surface -OH, O = C-O, and R-OH groups protonated and partly interact with Pb ions. Implying inner-sphere complex between lead ions and Fh-HA formed. The result also proved C loading can promote heavy metal binding to Ferrihydrite.
When Pb (II) adsorbed on ferrihydrite, hematite, or goethite, the Pb (II) ions have very similar coordination environments: distorted trigonal pyramidal coordination, with hydroxide ion ligands or surface oxygen ligands (Bargar J  ] sites. Pb(II) ions are adsorbed to in edgesharing manner on ferrihydrite, formed mononuclear sorption complex on FeO 6 octahedra. The reactions and adsorbate species formed are illustrated in Fig. 14. The Mössbauer spectra of Fh-HA after Pb adsorbed are shown in Fig. 13. Based on the data (Table 5), the Fe remains in a stable trivalent state, but Fe unit structure changed compared that before Pb adsorption. Decreased Qs implied a more symmetrical Fe unit structure. The original chemical bonds such as Fe-OH, R-COOH, and R-OH were replaced by Fe-O-Pb, R-COO-Pb, and R-O-Pb respectively, resulted in change of bond length, bond angle and bond energy. Based on the fact, the interaction between Pb(II) and Fh-HA is stronger than that between hydroxyl hydrogen and Fh-HA. In addition, site heterogeneity leads to the difference of complexation affinity and adsorption density; the combined effect contributed to the change of Fe unit structure.
The TEM images equipped with energy dispersive X-ray spectroscopy (EDX) of Fh-HA sample after Pb (II) sorption were determined. The element analysis from EDX (Fig. 12a) agrees well with the XPS survey spectra. The TEM images show the ferrihydrite has partly transformed, part converted to acicular goethite, part to spherical hematite particles as shown by arrows (Fig. 12b). As given in Fig. 12c, clear diffraction spots in the SAED pattern indicate the high crystallinity of goethite and hematite nanoparticles generated. A combination of TEM and SAED diffraction pattern suggests that with time, the Fh sample has transformed and tend to be more crystalline phase (Figs. 13 and 14). However, as showed in Fig. 11d, e, no obvious changes occurred on the TEM image and SAED pattern of Fh-HA precipitate sample; after a period of adsorption and desorption process, the ferrihydrite still dominated in mineral phase. The result implied the transformation of ferrihydrite has been inhibited when associated with HA.
It can be concluded that the Pb adsorption occurs on Fh-HA surface, and form Fh-HA-Pb ternary complex; this can explain why C loading in a certain range can enhance Pb sorption in spite of reduced SSA; new active sites from HA can balance the loss of SSA and even achieve a better complexation. Pb adsorbed on Fh-HA is expected to form steady mononuclear and binuclear bidentate complexes, and the extremely low Pb desorption amount agrees well the result. The sorption process is endothermic/exothermic, in Fh-HA-Pb ternary system, Original chemical bonds Fe-OH、R-COOH、and R-OH are replaced by Fe-O-Pb、R-COO-Pb、and R-O-Pb; chemical bond character has Fig. 13 Mössbauer spectra of ferrihydrite taken at room temperature after Pb(II) adsorption at various pH changed, such as bond length, bond angle, and bond energy. Two main sorption mechanisms happened in Fh-HA-Pb ternary system, electrostatic attraction and complexation. The outer-sphere complex formed by electrostatic attraction is unstable and easy to desorb, while the inner-sphere complex by complexation is very stable, not easy to be affected by the change of environmental conditions. The formation of Fh-HA-Pb ternary system greatly restrains the transformation of ferrihydrite. By the end of this experiment, in Fh-Pb binary system, ferrihydrite transformed to complex of ferrihydrite, goethite, and hematite, while in Fh-HA-Pb ternary system, ferrihydrite hardly transformed. In addition, humic acid can improve morphology stability of ferrihydrite by inhibiting crystalline growth and mineral transformation. The present researches conclude that low molecular organic ligands adsorbed to nucleation sites resulted in a slower crystallization of ferrihydrite (Liu and Huang 2003). Therefore, HA coprecipitated with ferrihydrite can be used as efficient and stable adsorbent for Pb ions. At the molecular level, the octahedral Fe 3+ distorted in the presence of humic acid and resulted in new geometric structure; the TEM with SAED images provide concerning proof for the assumption. Mineral transformation occurred on Fh with aging time, from poorly ordered state to more crystalline state, yielded goethite or hematite. However, which phase will be predominant in final product of ferrihydrite transformation has not been determined, considering that the measurement conducted before aging process finished. A few researches have indicated a close link between transformation product and pH condition during aging process of ferrihydrite (Kim, S.O. et al., 2014). The crystal model of ferrihydrite and transformation process is difficult to determine for poor crystallinity; new study (Funnell et al. 2020) uses RMC approach to construct and describe the two different models for ferrihydrite: multi-phase model (Drits et al. 1993) and single-phase model (Michel et al. 2007;Maillot et al. 2011). The paper concluded that the single-phase model is more accurate to describe 2-line ferrihydrite.

Conclusions
This paper discussed the coprecipitation properties of Fh with humic acid using XRD, BET, TEM, XPS, and Mössbauer spectra, further explored the mechanism of Pb adsorption onto Fh-HAs, and distinguished the changes Fig. 14 The reactions taking place during formation of Fh-HA coprecipitates, Pb(II) sorption on Fh-HA and evolution of ferrihydrite and aging of Fh and Fh-HA before and after metal ions adsorbed. Mössbauer and other characterization techniques results show that even small concentration of HA has a clear impact on surface, size, and structural properties on ferrihydrite. With increasing C/Fe, the interaction between HA and ferrihydrite is increasing, and stronger turbulence to early Fe 3+ octahedra structure. HA coprecipitated with HA affects the fate and mobility of lead ions in soils and water. The presence of HA results in increase in the available sites on the Fh surfaces and generates a higher negative surface charge, thereby possess a good dispersion in any interface and increase the Pb adsorption. Pb exhibits pH-dependent adsorption behavior and the maximum usually occur at neutral and alkaline pH.
In particular, the O1s XPS spectrum was investigated due to it consists of an effective molecular probe for sorption processes. A stable mononuclear and binuclear bidentate coordination between Pb ions and Fh-HAs formed. Aging process of ferrihydrite resulted in a mixed phase of ferrihydrite, goethite, and hematite. HA inhibited the transformation of ferrihydrite and kept morphology stability relatively. The study revealed how the varying amount of HA affected the ferrihydrite structure, the excellent properties of Fh-HA to adsorb Pb ions, and transformation product in aging process. These observations and conclusions would have enlightenment to application of Fh-HA in Pb-contaminated sites.