Simultaneous adsorption study of ammonium and phosphate by ZnFe-layered double hydroxide (ZnFe-LDH)-modified adsorbents.

Trace while cost-effective adsorption treatment is in prospect for clearing these pollutants simultaneously and efficiently. In this study, engineered adsorbents decorated with ZnFe-LDH by alkaline coprecipitation (3-6 mm, with modified ceramsite as CZF and modified volcanic rock as VZF) were prepared for NH 4 + and H 2 PO 4 - co-adsorption from aqueous solutions. Results Coexisting ions showed great effects on phosphate adsorption, especially Ca 2+ and CO 3 2- , and humic acid exhibited only a limited shielding impact on co-adsorption properties. Though comprehensive tests presented that the maximum captured amount of ammonium and phosphate occurred at pH 7, the isotherm and kinetic results suggest that two adsorbents preferentially adsorbed each pollutant. Specifically, the maximum uptake quantity of NH 4 + (15.55 mg-N g -1 ) and H 2 PO 4 - (11.21 mg-P g -1 ) were achieved by VZF and CZF, respectively, and controlled by physisorption and chemisorption distinctively, revealing that contaminants were cleared in disparate ways. Performances of seven consecutive adsorption/desorption cycles using 5% NaOH regenerant suggested that CZF retained 71.9% of PO 4 - removal efficiency, and VZF retained 47.9% of the NH 4 + adsorption rate, implying the strong reusability of ZnFe-LDH-modified adsorbents. A dynamic study has assessed that with 1 kg of combined modified adsorbents, approximately 527 L of polluted runoff flow could be continuously treated to below the National limits within one adsorption cycle. kinetics; (2) investigate the effects of interfering ions and organics, and pH on co-adsorption of ammonium (NH 4 -N) and phosphorus (PO 4 -P); (3) assess their sustainability and feasibility in eliminating eutrophication. Due to the prevalent use for water purification, porous matrices including ceramsite and volcanic rock were applied in this work as LDH-nanoparticle carriers. Combining spectroscopic techniques is expected to unravel the co-adsorption mechanisms of modified adsorbents. 4+ and H 2 PO 4− adsorption of modified adsorbents. In the studied pH range, both adsorbents exhibited the highest co-adsorption capacities at pH 7, with the total uptake quantity of 0.67 mg g − 1 by CZF and that of 0.73 mg g − 1 by VZF.


Background
Trace amounts of nitrogen and phosphorus can easily trigger eutrophication damage in surface water, while cost-effective adsorption treatment is in prospect for clearing these pollutants simultaneously and efficiently. In this study, engineered adsorbents decorated with ZnFe-LDH by alkaline coprecipitation (3-6 mm, with modified ceramsite as CZF and modified volcanic rock as VZF) were prepared for NH 4 + and H 2 PO 4 -co-adsorption from aqueous solutions.

Results
Coexisting ions showed great effects on phosphate adsorption, especially Ca 2+ and CO 3 2-, and humic acid exhibited only a limited shielding impact on co-adsorption properties. Though comprehensive tests presented that the maximum captured amount of ammonium and phosphate occurred at pH 7, the isotherm and kinetic results suggest that two adsorbents preferentially adsorbed each pollutant. Specifically, the maximum uptake quantity of NH 4 + (15.55 mg-N g -1 ) and H 2 PO 4 -(11.21 mg-P g -1 ) were achieved by VZF and CZF, respectively, and controlled by physisorption and chemisorption distinctively, revealing that contaminants were cleared in disparate ways. Performances of seven consecutive adsorption/desorption cycles using 5% NaOH regenerant suggested that CZF retained 71.9% of PO 4 -removal efficiency, and VZF retained 47.9% of the NH 4 + adsorption rate, implying the strong reusability of ZnFe-LDH-modified adsorbents. A dynamic study has assessed that with 1 kg of combined modified adsorbents, approximately 527 L of polluted runoff flow could be continuously treated to below the National limits within one adsorption cycle.

Conclusion
Comparing with other parallel adsorbents, ZnFe-LDH-modified adsorbent is promising in eliminating eutrophication due to their superior capacity, stability, renewability, and non-toxicity.

Background
Eutrophication of aquatic systems is commonly caused by the unregulated discharge of agricultural irrigation water and domestic sewage that rich in nitrogen and phosphorus. Generally, their superfluous presence beyond threshold values in surface runoffs will mark stimulatory impacts on non-point eutrophication (above 2 mg L − 1 for ammonium (NH 4 -N) and 0.4 mg L − 1 for phosphorus (TP)) [1]. As a country dominated by the agricultural industry, China is in large uses of fertilizer containing nitrogen and phosphorus, posing a heavy burden in surface water regulation. Prevalent treatment approaches involve struvite generation [2], biofilm [3], biodegradation [4], and reverse osmosis [5], etc. However, their synchronous removal of ammonium and phosphorus at low concentrations is physically or technically limited in treating non-point polluted cases [6].
In tackling this issue, adsorption treatment has been developed as a sustainable and versatile technique that can be employed in real runoff depollution [7,8]. Recently, various porous materials have becoming extensively investigated in co-adsorption of ammonium and phosphorus due to their wide availability, facile fabrications, and serving as sorbents by themselves, such as biochar [2,9], wheat straw [10], clinoptilolite [11,12], diatomite [13], and red mud [14]. Through proper metalfunctionalization, their efficiency, selectivity, and reusability have been promoted considerably compared with pristine filter materials. Among diverse modifications, iron-decoration is regarded as the most promising one due to its high adsorption capacity and environmental friendliness [15,16].
Layered double hydroxides (LDHs) are positively charged materials structured as ordered octahedron and composed of metal hydroxides with anions and water molecules intercalated for charge balance [17]. In its typical expression [N 2+ (1−x) N 3 + x (OH) 2 ] x+ [A n− ] x/n •mH 2 O, N 2+ and N 3+ serve as bivalent and trivalent metal ions, respectively, and A n− are exchangeable anions [18]. The symbol x describing the metal ratio (x = N 3+ /(N 2+ + N 3+ )) is mainly in the range of 0.2-0.33 [19]. The stunning affinity to phosphate due to the weak interlayer bonding makes LDHs a promising adsorbent in wastewater purification [20][21][22]. Researchers have proved that ZnFe(III)-based oxides/hydroxides own tremendous capacity for oxyanions scavenge [23][24][25][26]. Also, a screening study proposed that Cl − as the exchangeable anions better benefited the LDH adsorption performance than other intercalated substances [27]. Therefore, ZnFe(III)-Cl-LDH stands out to be the most favorable metal precursor for porous adsorbent functionalization.
Given these backgrounds, the current study aims to: (1) compare the co-adsorption performance and preferential adsorption of ZnFe-LDH-modified adsorbents in terms of adsorption isotherms and kinetics; (2) investigate the effects of interfering ions and organics, and pH on co-adsorption of ammonium (NH 4 -N) and phosphorus (PO 4 -P); (3) assess their sustainability and feasibility in eliminating eutrophication. Due to the prevalent use for water purification, porous matrices including ceramsite and volcanic rock were applied in this work as LDH-nanoparticle carriers. Combining spectroscopic techniques is expected to unravel the co-adsorption mechanisms of modified adsorbents.
ZnFe-LDH-modified adsorbents were prepared following an established alkaline coprecipitation method [28]: an amount of clean pristine materials were homogeneously mixed with a 500 mL aqueous mixture of FeCl 3 ·6H 2 O and ZnCl 2 (Zn:Fe = 2:1) in a 1L beaker. To the solution was dropwise added 5 M NaOH to stabilize the pH at 11-12 with vigorous stirring for 4 h at 85℃. After thorough precipitation, the solid phases were separated and rinsed to neutral pH with Milli-Q water, and thereafter oven-dried at 60℃ for 16 h. Digital photos of modified adsorbents in Fig. S1 didn't show an obvious loss in the resultant size but conspicuous changes in outlook.

Adsorption experimental section
Batch adsorption experiments were following the pattern unless being specifically stated: in 150 mL flasks with 100 mL liquor and adsorbent dosage of 20 g L − 1 contained, reactions were carried out at 120 rpm, pH initial =7, and 25 ± 1 ℃ for 48 h using a thermostatic shaker to ensure equilibrium. The

Evaluations in reusability and continuous flow adsorption
The reusability of adsorbents was assessed on the exhausted adsorbents collected from the pH investigation system (pH = 7). The pollutants-loaded adsorbents were fully rinsed with Milli-Q water to clear away any weakly attached pollutants. Based on the negative performance of adsorbents showed in alkaline media and the exchangeability of sodium hydroxide proposed in a previous paper [29], regeneration was conducted using 5 w/v % NaOH (pH = 13) and thoroughly extracted for 12 h at 25℃.
After desorption, the solid phases were completely rinsed with Milli-Q water and oven-dried for 4 h before reuse. . pH pzc and CEC measurements of modified adsorbents were described in Supplementary information SI, Section S1-S2.

Statistical analysis
In this study, uptake capacities and the best-fits to isotherm and kinetic models were calculated with the calculation formulae listed in SI. Section S3, parameters of kinetic and isotherm curves were generated by Auto2Fit software. Experimental data of the influences of interfering ions were statistically analyzed via analysis of variance (ANOVA) by SPSS version 18.0.

Results And Discussion
Systematic characterizations of modified adsorbents SEM analyses for the morphologies of adsorbents before and after ZnFe-LDH-modification are presented in Fig. 1. Dense coating of ZnFe-LDH chips has roughened the surface of CZF (b-c) and VZF (e-f) in comparison with the even outlook of the original ceramsite (a) and volcanic rock (d).
Meanwhile, higher porosity and the overlapping texture exhibited on modified adsorbents indicate an upscaled heterogeneity. As shown in Fig. S2 The key physicochemical properties of modified adsorbents were listed in Table 1. Notably, CZF (5.3 m 2 g − 1 ) and VZF (3.7 m 2 g − 1 ) obtained higher specific surface area (SSA) values than their original ones (1.6 m 2 g − 1 and 2.5 m 2 g − 1 ), which were benefited from the introduced nanoparticles with high SSA. As shown in Fig. S3, the N 2 sorption/desorption isotherms of prepared samples exhibited H3 hysteresis loops in type IV isotherms, especially VZF, implying the slit-shaped pores of packing plate-like particle and the occurrence of N 2 physical adsorption on heterogeneous interfaces [30]. Such an escalation in heterogeneity as also being corroborated by the increased pore volume and pore radius should be ascribed to the built-in modification that provided more porosity and ion exchange channels for targeted adsorbates (Table 1) [31]. Additionally, the suggested pH pzc values of CZF (8.90) and VZF (8.79) were substantially elevated compared with the ZnFe-LDH nanoparticle (6.4) [32]. When the liquid pH is below the proposed pH pzc , the solid will be positively charged through metal hydroxide protonation (Eq. 1-2). Thus, phosphorus removal will be enhanced by electrostatic attraction due to the charge complementarity [33]. Particularly, VZF was of a greater CEC property, which is in agreement with its higher mass proportion of Na + in the EDS result and higher Na + content that noticed in the chemical composition, and is expected to benefit the NH 4 + capture.
Meanwhile, the remarkable increase in the contents of Zn and Fe compounds after modification suggests the successful LDH-functionalization.

Comparison of modified adsorbents in kinetics and isotherms studies
The intake performances of original and modified adsorbents in sole and binary-pollutant media were displayed in Fig. S4. The obvious increments of modified adsorbents in NH 4 + and H 2 PO 4 − sorption capacities compared with the unmodified suggest that ZnFe-LDH modification has improved the synchronous removal efficiency. Meanwhile, the co-uptake behaviors in binary systems witnessed insignificant fluctuations in contrast to those in sole systems, implying the bifunctionality of modified adsorbents won't weaken during co-adsorption.
Further insights into the co-adsorption processes of modified adsorbents were collected in Fig. S5a.
Specifically, the co-adsorption capability of VZF (1.21 mg g − 1 ) was marginally higher than that of CZF (1.18 mg g − 1 ). In comparison, CZF showed a higher equilibrium capability of 0.45 mg-P g − 1 for phosphorus adsorption, and VZF exhibited a greater efficiency of 2.14 mg-N g − 1 for ammonium removal, indicating the preferential adsorption by different adsorbents. The controlling mechanisms of NH 4 + and H 2 PO 4 − adsorption were clarified by three kinetic models ( chemisorption, which involves around complexes formation, sharing electrons, and anion intercalation [9].
As the isotherms depicted in Fig. S5b respectively. The highest affinity for targeted contaminants reported by K f was deduced from the Freundlich isotherm, whose n values of all adsorption processes were higher than 1, demonstrating favorable adsorption performances [33]. Therefore, the multilayer adsorption was dominant during the NH 4 + and H 2 PO 4 − capture processes. In this study, the maximum sorption capacity of modified adsorbents for synchronous removal of N and P was greater than the various adsorbents reported in Table S2, leading to their strong competitiveness.

Mechanisms in simultaneous adsorption
To unveil the adsorption mechanisms responsible for simultaneous uptake of phosphorus and ammonium by ZnFe-LDH-modified adsorbents, both fresh and pollutants-saturated adsorbents were collected, followed by characterization with XPS. Compared with the ones before pollutants interactions, the full scan of modified adsorbents after adsorption shows the disappearance of the Cl 2p and Na 1 s core peaks, and new P 2p and N 1 s peaks occurred at 134.0 eV and 400.2 eV, respectively (Fig. S6a), indicating that ion exchange controlled the N and P capture to some extent.
As shown in Fig. 2a, two peaks at 134.1 eV and 133.7 eV were correlated with the typical HPO 4 2− and H 2 PO 4 − [34]. Thus, P was mainly removed as anions through ion exchange and electrostatic attraction rather than being immobilized by precipitating as Zn-PO 4 or Fe-PO 4 . As noted in Fig. 2b, two peaks at 400.5 eV, and 406.8 eV correspond to NH 4 + in different forms [11], which were retained by exchanging with the inherent Na + , as can be demonstrated by the diminished Na percentages. The Zn 2p 3/2 peak (1022.8 eV) and the Fe 2p 3/2 (711.3 eV) peak with their shakeup satellites before and after adsorption were shown in Fig. S6b [32]. Notably, no shift has occurred to the Zn element; the upward shift in the Fe 2p peak (0.2-1 eV), however, suggests that the fixation of P can be speculated to form at the ligand shell of Fe atoms. Thus, Fe has a direct affinity for P, while the Zn atom served as the architecture supporter ensuring the LDH stability upon phosphate interaction [35].
Overall, the mechanisms behind the co-capture of ammonium and phosphate by ZnFe-LDH-modified adsorbents can be elucidated in Fig. 3 [8,36]. Nonetheless, VZF's stable uptake of NH 4 + when confronting interfering cations marked its selectivity to NH 4 + . For phosphorus removal, Ca 2+ was the most favorable (p < 0.001), Mg 2+ had negligible effects, and the rest of the ions (Na + , NO 3 − , SO 4 2− , and CO 3 2− ) have obvious shielding effects on the amounts of P captured (p < 0.001). Ca-P amorphous precipitates with low K sp values tend to be intercepted by the host pore structure [37], thus promoting phosphorus removal. It is comprehensible that oxyanions inhibited phosphorus removal by competing for LDH's anion exchange capacity and electrostatic adsorption sites [31]. The dramatic dive in P ingestion in the presence of CO 3 2− was ascribed to carbonate's priority into the LDH interzone and due to the OH − -induced competitive adsorption resulted from its hydrolysis (k h =2.12 x 10 −4 ) [38]. Inhibition rendered by background electrolytes (Na + ) can be interpreted by its competition with the electrostatically-adsorbed phosphate on the outer-sphere adsorption sites [39]. Such ions-dependent adsorption indicates that phosphorus removal was mainly attributed to the nonspecific interactions brought about by LDH. In summary, better co-adsorption quantities observed in VZF suggest a more stable performance of VZF when confronting with extraneous salt interference.
Despite the truth that coexisting ions engendered heavier effects on phosphate adsorption than ammonium capture, the mechanically sturdy hosts in this study endowed LDH architecture with integrity to reduce much interference, as compared with the ion-sensitivity showed in the previous study [31].
Humic acid (HA) is a typical dissolved organic matter in eutrophic water bodies. However, HA showed shielding effects on NH 4 + and H 2 PO 4 − co-adsorption in the current practice (Fig. 4b). As the HA intensity ascended from 2 to 5 mg L − 1 , a limited loss was engendered in phosphate removal (16-24% for CZF and 0-1% for VZF), and no significant change took place afterward. This is consistent with a preceding study [21]. Negatively charged HA reduced specific phosphorus adsorption by forming inner-sphere complexes on the LDH [40]. Therefore, the increase of HA intensity won't further suppress the nonspecific phosphorus adsorption. For ammonium ingestion, the inhibitory effect was slight (3-16%) when the HA intensity was below 40 mg L − 1 ; after that, however, ammonium removal of VZF descended drastically (47%). This can be explained that NH 4 + could electrostatically attach to organics and then remain in solutions [41]; thus, the ammonium abatement was obstructed as HA increased. To sum up, the shielding effect of HA was weaker when capturing phosphate than when holding ammonium. Thanks to the robust phosphorus removal of LDH structure, CZF achieved better co-uptake under dramatic HA interference. Compared with other LDH-adsorbents [42], this work has faced down the issue of HA interference on phosphorus due to the fixing advantage of the host materials. Further investigation of metal leaching in the presence of HA is required.

Influences of pH on simultaneous adsorption
As can be seen in Fig. 5, pH has significant influences on the NH 4 + and H 2 PO 4 − adsorption of modified adsorbents. In the studied pH range, both adsorbents exhibited the highest co-adsorption capacities at pH 7, with the total uptake quantity of 0.67 mg g − 1 by CZF and that of 0.73 mg g − 1 by VZF.
Meanwhile, NH 4 + removal efficiencies of both adsorbents crested at pH 7 (29.2% for CZF, 56.9% for VZF), and gradually reduced when pH turned to acidic or alkaline regions. This scene is aligned with the previous finding [43]. In alkaline liquid systems, cationic NH 4 + (pK a =9.25) would convert into NH 3 ( Fig S6a), a non-protonated form that resists cation exchange [44]; while in acidic solutions, the increased concentration of H + would enhance the competition with NH 4 + , causing suppression on ammonium intake [44]. Additionally, both adsorbents worked better in removing phosphate at pH 3 (100.0% for CZF, 65.0% for VZF), and VZF shifted its preference from ammonium to phosphate when pH < 6. As shown in Fig. S7b, the species of phosphorus in the neutral medium were mainly H 2 PO 4 − (pK 1 = 2.15) and HPO 4 2− (pK 2 = 7.20), which are favorable for the phosphorus adsorption [8].
Whereas phosphate adsorption in the alkaline region was reduced by the electrostatic repulsion of negatively charged adsorbents when pH > pH pzc and by the adsorption competition between OH − and anionic phosphate [45]. Overall, pH has heavier influences on the capture process of NH 4 + than that of H 2 PO 4 − . In contrary to recent research [31], the robust P intake of CZF over such a wide pH range can be attributed to the anti-interference properties that the original host provided. In this way, CZF exhibited better adaptation than VZF under pH fluctuations.
As can be seen in Table S3, the released amount of Zn 2+ (0.0017-0.0089 ppm) after adsorption at pH 7 were below the toxic thresholds for aquatic systems (0.05 ppm) and plants (0.01-0.13 ppm) [1,46], and farther beneath the National limits of dangerous wastes (GB 5085. . Meanwhile, the released Fe 3+ was even more negligible. The above results demonstrated that ZnFe-LDH-modified adsorbent could satisfy most natural circumstances.

Regeneration cycles and continuous flow adsorption test
Considering the ever-increasing calls for the regeneration strategy, seven consecutive adsorption/desorption cycles were performed on current adsorbents via 5% NaOH regenerant. As depicted in Fig. 6, all sequential cycles noticed a limited drop in the first two cycles and plateaued from cycle 3. Throughout the cyclic adsorption, CZF and VZF retained robust decontamination efficiency for H 2 PO 4 − (71.9%) and NH 4 + (47.9%), respectively, while underperformed in their nonpreferential pollutants (12.7%). Meanwhile, desorption cycles exhibited in Fig. S8 found the constant P desorption performance (> 93.0%), indicating that the nonspecific P adsorption could be reversed and renewed by abundant hydroxyl ions. The irreversible desorption portion was attributed to specific adsorption that formed strong metal-P bonding. Compared with parallel studies [47,48], the repeated chemical desorption in this study didn't perform significant impair in performance and weight aspects of the modified materials, suggesting their strong reusability in the potential engineering practice.

Conclusions
Here, ZnFe-LDH-decorated adsorbents based on ceramsite and volcanic rock were prepared for the simultaneous removal of ammonium and phosphorus. Despite superior adsorption capacities in comparison with various adsorbents, CZF and VZF showed the corresponding affinity for H 2 PO 4 − (11.21 mg-P g − 1 ) and NH 4 + (15.55 mg-N g − 1 ), respectively. Among studied influencing factors, coexisting ions (Ca 2+ and CO 3 2− ) have greater effects on phosphate adsorption, while pH and humic acid interfered more with ammonium adsorption. Correspondingly, CZF behaved better in most scenarios. The XPS analysis proved that ammonium was mainly removed by the cation exchange of raw materials, while phosphorus was taken up by the coated LDH through anion intercalation, electrostatic attraction, and ligand exchange, thus the two occupied distinct ways during copurification. Overall, due to the low metal leaching, renewability, and practicality in dynamic flow indicate that the ZnFe-LDH-modified adsorbent is a promising candidate for eutrophication mitigation.    Schematic mechanism of simultaneous adsorption by ZnFe-LDH-modified adsorbent.

Figure 4
The effects of coexisting ions (a), and humic acid (b) on co-adsorption.

Figure 5
The effects of pH on co-adsorption by ZnFe-LDH-modified adsorbents.

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