Journal of Water Process Engineering 55 (2023) 104218 Contents lists available at ScienceDirect Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe Outstanding performance of hierarchical alumina microspheres for boron removal in the presence of competing ions Xiang-Yang Lou a, Roberto Boada a, *, Lucia Yohai a, b, Manuel Valiente a a GTS-UAB Research Group, Department of Chemistry, Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain b Instituto de Investigaciones en Ciencia y Tecnología de Materiales (INTEMA, UNMdP-CONICET), B7608FDQ Mar del Plata, Argentina A R T I C L E I N F O A B S T R A C T Keywords: Developing efficient materials for the removal of boron from aqueous solutions is becoming an important task to Boron overcome boron pollution. Herein, we present hierarchical alumina microspheres (HAM) as an outstanding Adsorption adsorbent, synthesized via a microwave-assisted co-precipitation method. The microstructure, morphology, and Gamma-phase hierarchical alumina textural characterization of the HAM particles carried out by X-ray diffraction (XRD), scanning electron mi­ microspheres Competing ions croscopy (SEM), and transmission electron microscopy (TEM) revealed hollow γ-Al2O3 particles with a porous Regeneration dandelion-like shape and an average size of 1.5 μm. The analysis of the adsorption data indicated that the adsorption was homogeneous in a single layer and that chemical adsorption was the controlling step in the process. The adsorption capacity obtained at an initial concentration of 800 mg⋅L− 1 was 51.60 mg⋅g− 1, and the theoretically calculated maximum adsorption capacity using the Langmuir model was 138.50 mg⋅g− 1, which outperforms previously reported adsorbents. The determination of thermodynamic parameters indicated that the adsorption is an exothermic and non-spontaneous process. The XPS spectra of HAM after adsorption indicated the formation of Al-O-B bonds. Of particular interest for industrial applications, the HAM adsorbent showed excellent selectivity for boron in the presence of competing cations or anions and at different ionic strengths. In addition, HAM maintained a high adsorption capacity after five consecutive adsorption/desorption cycles. These findings highlight the potential of HAM as a highly microporous material for boron removal in real industrial applications. 1. Introduction during desalination procedures, the development of effective boron remediation processes has become an important task [10]. Boron (B) is an essential micronutrient for plants and it is released The presence of different chemical boron species in water and their into the environment from both natural processes (e.g., weathering of concentration variability imposes a great challenge for achieving an rocks and leaching of salt deposits) and anthropogenic activities (e.g., efficient and selective boron removal from polluted water [11]. the production of glass, porcelain, semiconductors) [1,2]. The average Compared with other remediation methodologies, adsorption is boron concentration found in the environment varies strongly depend­ considered to be one of the most promising approaches owing to its ing on the media: 9–85 mg⋅kg− 1 in soil [3], 4.5 mg⋅L− 1 in seawater [4], numerous advantages such as high efficiency, easy operation, environ­ 0.3–100 mg⋅L− 1 in groundwater [5], 0.01–200 mg⋅L− 1 in surface-water mentally friendly, low cost, and suitability for the removal of low con­ [6], and higher than 100 mg⋅L− 1 in sewage wastewater [7]. Unfortu­ centration pollutants [12]. Various traditional adsorbents, such as nately, the presence of high concentrations of boron causes environ­ activated carbon [13], fly ash [14], silica [15], biomass [16], and mental (e.g., acid rain and crop poisoning) and human health issues (e. magnetic nanobeads [17] have been used for boron removal. The main g., kidney damage, anorexia, and diarrhea) [8]. Indeed, the World advantages of these adsorbents are their low cost and easy accessibility. Health Organization (WHO) has established a guideline value of 2.4 However, they have low adsorption capacities (<20 mg⋅g− 1) and bad mg⋅L− 1 as the upper limit for boron in drinking water [9]. Taking into selectivity due to the weak interactions between the adsorbate and the account the toxicity of concentrated boron in groundwater and surface adsorbents [18]. Many polyhydroxy-functionalize commercial resins water, and the need for removing boron from wastewater and seawater have been also used for B removal because they show a high boron * Corresponding author. E-mail address: roberto.boada@uab.cat (R. Boada). https://doi.org/10.1016/j.jwpe.2023.104218 Received 5 April 2023; Received in revised form 24 August 2023; Accepted 24 August 2023 Available online 31 August 2023 2214-7144/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). X.-Y. Lou et al. Journal of Water Process Engineering 55 (2023) 104218 selectivity [13], however, their limited chemical and thermal stability, scanning electron microscope, SEM (Merlin, Carl Zeiss AG, Oberkochen, together with their low adsorption capacities (e.g., Amberlite PWA10, Germany) and transmission Electron Microscope, TEM (JEM-1400, LSC 780, Diaion CBR05, and Purolite S108 have an adsorption capacity JEOL, Tokyo, Japan). The SEM images were analyzed using Image-pro within 5.9–7.2 mg⋅g− 1) prevent them to be considered as viable adsor­ Plus software to determine the particle size distribution. X-ray powder bents for B remediation [19]. Besides, raw materials used for synthe­ diffraction, XRD (X-Pert, Philips, Amsterdam, Netherlands) was carried sizing these resins are expensive, and the cost of recycling those out to identify the crystallographic phase of HAM. Fourier-transform materials is also rather high which are additional drawbacks to infrared spectroscopy, FTIR (Nicolet iS10, Waltham, America), and X- overcome. ray photoelectron spectroscopy, XPS (Thermo Scientific K-Alpha, Wal­ In this study, we propose using hierarchical gamma-phase alumina tham, America) analysis were performed to study the adsorption microspheres (HAM) synthesized via microwave-assisted co-precipita­ mechanism. tion method as an alternative adsorbent for achieving effective and se­ The point of zero charge (pHpzc) of HAM was detected using the pH lective boron removal from aqueous solutions in the presence of drift method [26]. Various flasks were filled with 30 ml of 0.01 M KNO3 competing ions. The high density of hydroxyl groups on the surface of solutions. The pH of these solutions was adjusted to get a pH from 2.0 to γ-Al2O3 provides abundant adsorption sites for the effective removal of 10.0 using 0.1 M HCl and NaOH solutions. Subsequently, 0.1 g of pollutants from water, which has been demonstrated in numerous adsorbent was added to each solution, and the pH was measured again studies [20,21]. Besides, in contrast with the commercial resins after 48 h. The pHpzc is determined as the pH value at which the initial mentioned above, the low-cost and environmentally-friendly production and final pH values are identical. makes HAM a more sustainable and economically viable option for large-scale water treatment applications. Furthermore, the overall 2.4. Adsorption and desorption experiments micrometer-sized structure of γ-Al2O3 particles provides several ad­ vantages over other nanoscale adsorbents. It reduces the aggregation of Boron adsorption experiments were performed in 50 ml plastic tubes particles in solution, which enhances their dispersibility and prevents containing 25 ml of boron solution and 0.1 g of adsorbent. The tubes the formation of filter cake during the filtration process. This, in turn, were agitated mechanically at 300 rpm, and the experiments were results in better mechanical strength, facile transportation, and easy performed at different temperatures: 293, 318, and 343 K. The solutions recovery of the adsorbent, making it a practical option for real-world were separated from the adsorbents by centrifugation (3000 rpm, 5 water treatment applications [22,23]. min), and the concentration of the unabsorbed boron was determined by the carmine colorimetric UV–Vis method (see Section S2 for further 2. Materials and methods details). The adsorption capacity at equilibrium (qe) was calculated using the following equation: 2.1. Chemical reagents V qe = (Co − Ce ) (1) m All chemicals used in the experiments were of analytical grade and purchased from Sigma-Aldrich (Missouri, USA), Panreac (Barcelona, where qe (mg⋅g− 1) is the adsorption capacity, V (L) is the volume of the Spain), Scharlab (Barcelona, Spain), or Honeywell, (Charlotte, USA). solution, m (g) is the weight of the adsorbent, Co and Ce (mg⋅L− 1) are Aluminum potassium sulfate dodecahydrate (KAl(SO4)2⋅12H2O) and initial and equilibrium concentrations of boron, respectively. urea (CO(NH2)2) were used to synthesize HAM. The source of boron was Desorption experiments were performed using 25 ml of 0.1 M HCl boric acid, B(OH)3. An aqueous stock solution of 1000 mg⋅L− 1 boron was solution and agitating for 2 h. The adsorbents were separated by first prepared and then diluted as required. Hydrochloric chloride (HCl) centrifugation and the adsorption/desorption cycle was repeated several and sodium hydroxide (NaOH) were used to adjust the pH of the solu­ times to assess the regeneration capabilities. tions in the synthesis and adsorption processes. Carmine (C22H20O13) and sulfuric acid (H2SO4) were used for the detection of boron by the 2.5. Effect of competing ions and different ion strength carmine colorimetric method as reported elsewhere [24]. The method is described in detail in Section S1 in the Supplementary information (SI). To examine the competing effects in the boron adsorption procedure due to the presence of other ions that may be present in natural systems, 2.2. Microwave-assisted synthesis of HAM a 1:1 molar concentration mixture of each ion (cations or anions) and boron was used with an initial concentration of 18.50 mM (200 mg⋅L− 1 HAM was synthesized using a microwave (MARS-5, CEM, Matthews, for boron). This concentration was chosen considering the boron con­ North Carolina, USA) assisted methodology following previously re­ centration in real wastewater of oil/gas excavation, geothermal water, ported works [25]. In brief, a 100 ml aqueous solution of aluminum mining, metal-processing, or semiconductor industries as reported by potassium sulfate dodecahydrate (0.05 M) and urea (0.10 M) was stirred Lin et al. [6]. Besides, to account for the composition of wastewater in at 300 rpm for 15 min. The resulting solution was divided into seven real conditions, experiments with a higher concentration ratio of microwave vessels of 100 ml using a graduated cylinder, then heated at different anions ([B]:[anion], 1:100), were also performed. Different salt 180 ◦ C for 20 min setting the microwave power to 1000 W. Once cooled reagents were used as cation sources (NaCl, KCl, CaCl2, MgCl2), metal down to room temperature, the pH was adjusted to ~9.0 by adding 1 M sources (Ni(NO3)⋅4H2O, Cu(NO3)2⋅6H2O, Cr(NO3)2⋅6H2O and Fe NaOH solution (the initial pH of the solution was ~7.2). The resulting (NO3)2⋅6H2O) and anion sources (NaCl, NaNO3, Na2SO4⋅10H2O, solid material was collected by centrifugation (3000 rpm, 5 min) and Na3PO4⋅12H2O). The adsorption experiments were performed at pH 8.0. washed with hot Milli-Q water (70 ◦ C) and ethanol to remove the excess To examine the effect of the ion strength of the solution on the boron chemicals. The collected precipitate was dried in an oven at 80 ◦ C for 12 adsorption capacity, a series of adsorption experiments were performed h. The dried precipitate was calcinated in a muffle at 600 ◦ C for 2 h. After by adding NaCl to the boron solution within the range 0–58.44 g⋅L− 1. To cooling down to room temperature, the lump of material finally ob­ further examine the competitive adsorption of boron under real condi­ tained was broken down by gently grinding and stored for future use. A tions, simulated tap water and ground water polluted with boron were diagram of the HAM synthesis is displayed in Fig. S1. prepared. The compositions of these samples are provided in Section S3 and Table S1. 2.3. Characterizations of HAM The morphology and size of the HAM particles were studied by 2 X.-Y. Lou et al. Journal of Water Process Engineering 55 (2023) 104218 3. Results and discussion Regarding boron, at pH values below 7.0, it exists predominant as B (OH)3, which exhibits low affinity to HAM due to the weak Lewis acid 3.1. Characterization of HAM nature of boric acid. As the pH increased to 9.0, B(OH)3 is converted into B(OH)−4 rapidly. These borate anions can interact favorably with amino The morphology study of HAM provided information about the and hydroxyl groups [31]. Despite the electrostatic repulsion between shape, average size, and size distribution of the synthesized particles. the negative surfaces of both HAM and B(OH)−4 , the dominant ion ex­ SEM images (Fig. 1a) showed that HAM are hollow particles with a change of boron anions with hydroxyl groups on HAM surface led to a highly textured surface and porous dandelion shape. This structure higher adsorption capacity of boron. At pH values above 9.0, the provides a large specific surface area (ca. 250 m2⋅g− 1) [25], which chemical form of boron was B(OH)−4 , and the strong electrostatic would benefit the boron adsorption. The average size of HAM particles repulsion between HAM and B(OH)−4 prevailed, leading to a decrease in was 1.5 ± 0.5 μm. These values are similar to those reported in previous the adsorption capacity [32]. works following a similar synthesis procedure [25]. The hollow and porous dandelion morphology of the HAM particle was confirmed in the 3.3. Effect of contact time TEM image (Fig. 1b). Besides, the size of HAM determined from TEM images was consistent with the results from SEM characterization. In The adsorption kinetic parameters are important to determine the addition, XRD was used to identify the crystal structure of the synthe­ efficiency and the mechanism responsible for the adsorption process sized HAM particles. The diffraction pattern obtained, Fig. 1c, corre­ [33]. In this study, different contact time experiments from 1 min to 24 h sponds to the structure reported for γ-Al2O3 (Crystallography Open with 200 mg⋅L− 1 initial concentration, 293 K reaction temperature and Database ID: 2015530) [27]. Even though the diffraction peaks of the pH 8.0 were performed. The results displayed in Fig. 2b revealed an HAM particles are rather wide due to the nanosized wall thickness of the initial sharp increase in the adsorption capacity, which might be hierarchical structure, the main reflections are matching those obtained attributed to a large number of adsorption sites initially available on the from the simulation of the γ-Al2O3 structure. surface of HAM. After 120 min, the adsorption capacity becomes almost constant, which means that the adsorption equilibrium has been 3.2. Effect of pH reached. To better study the rate and mechanism of boron adsorption onto the In water, boron speciation is influenced by the pH and the concen­ HAM surface, the experimental kinetic data were fitted to the pseudo- tration of the solution (see Fig. S2). The most common species of boron first order kinetic model and pseudo-second order kinetic model. The are boric acid, B(OH)3, and various kinds of borates (e.g., B(OH)−4 ) [28]. equations of those two models are displayed in Section S6. The modeling Boric acid and borate oxyanion, B(OH)−4 , are mainly present at low results are shown in Fig. 2c and the parameters obtained from simulated concentrations (<216.2 mg⋅L− 1). Boric acid dominates at low pH values, curves are shown in Table 1. Despite obtaining a relatively good while borate ions dominate at high pH values. At high concentrations agreement with the two models (R2 > 0.90), the pseudo-second order (>270.3 mg⋅L− 1), water-soluble polyborate ions such as B2(OH)−5 , kinetic model provided a better agreement. This implies that chemi­ B3O3(OH)−4 , and B4O5(OH)2− 4 are formed with an increase of pH value sorption is controlling the velocity of the adsorption process. Hence, the from 6.0 to 10.0 [29]. Therefore, pH is an important parameter that adsorption of boron on HAM involves valence forces and ion exchange should be investigated when studying boron adsorption [30]. The boron mechanisms through the sharing/exchange of electrons between the adsorption capacity of HAM with 200 mg⋅L− 1 initial concentration, 0.1 g hydroxyl groups on the adsorbent surface and boron [34,35]. of the adsorbent per 25 ml solution, 120 min contact times at 293 K as a function of pH is shown in Fig. 2a. 3.4. Effect of different initial concentrations The results show that within the pH range studied, the adsorption capacity increases with the pH until reaching a plateau around 6.0–7.0 In our work, adsorption experiments at different initial boron con­ which is followed with a further increase reaching a maximum (14.92 centrations from 1 to 1000 mg⋅L− 1, 120 min contact times at 293 K and mg⋅g− 1) at around 8.0. For higher pH, a slight decrease is found. These pH 8.0 were performed. As can be seen in Fig. 2d, the adsorption ca­ results are related to the change in the fraction of the boron species pacity increases monotonically with the initial concentration of boron present as a function of pH and influenced by the charge of the HAM without reaching saturation within the concentration range studied. It is surface that varies as a function of the pH. As reported, alumina has worth mentioning that the top-end of the concentration range is much considerably strong Lewis acidity and little Brönsted acidity [22]. The higher than the concentration found in polluted waters targeted by this pHPZC of HAM was determined by the pH drift method. Fig. S3 shows study, ~200 mg⋅L− 1 [6]. that the pHpzc of HAM is 5.9. Therefore, at pH values below 5.9, the The adsorption isotherm modeling results are displayed in Table 2. surface of HAM is positively charged, whereas at pH values above 5.9, Among the three models considered (Langmuir, Freundlich, and Tem­ the surface is negatively charged. kin; see Section S7), Langmuir and Freundlich models provided a good Fig. 1. Morphological and structural characterizations of synthesized HAM: SEM images (a), TEM images (b), and XRD pattern (c). 3 X.-Y. Lou et al. Journal of Water Process Engineering 55 (2023) 104218 Fig. 2. Effect of different parameters on boron adsorption capacity of HAM: pH values (a), contact time (b), kinetic fitting (c), initial concentration with corre­ sponding isotherm modeling (d). obtained from the linear fit of the Arrhenius plot (see Fig. S4, Table 3). Table 1 See Section S8 for further details. Our results show that ΔH◦ is negative, Kinetic parameters obtained from the model fitting of boron adsorption on HAM: which indicates the adsorption of boron by HAM is an exothermic pro­ K1 (min− 1), K2 (g⋅mg− 1⋅min− 1). cess and prefers to happen at lower temperatures. However, for 293, Kinetic model R2 Rate constant 318, and 343 K, ΔG◦ was determined to be 5.18, 8.15, and 11.11 Pseudo-first order 0.9795 K1 = 0.0290 kJ⋅mol− 1, respectively. The positive values of ΔG◦ for all investigated Pseudo-second order 0.9996 K2 = 0.0445 temperatures show the non-spontaneity of boron adsorption using HAM. Besides, the value of ΔG◦ increases by increasing the temperature which agreement (R2 > 0.99), however Langmuir model was the best one. This itself shows that higher temperatures result in a decreasing in the boron suggests that chemisorption is the main process [36–38]. Besides, for the adsorption capacity. In addition, a decrease in randomness at the solid- Temkin model, the value of the regression coefficient is much worse, R2 solution interface during the adsorption process was indicated by the = 0.6993, which means that the boron adsorption process by HAM negative value of ΔS◦ [39]. cannot be explained by this model. This points out that, in our case, the heat of adsorption does not decrease linearly with the decrease of sur­ 3.6. Effect of the competing ions face coverage. The obtained adsorption capacity at 800 mg⋅L− 1 initial concentration was 51.60 mg⋅g− 1 and the theoretical maximum capacity To investigate the effect of competing ions on the boron adsorption calculated by the Langmuir model was 138.50 mg⋅g− 1 which is an by HAM, several boron adsorption experiments were performed by outstanding value when compared with other reported materials as it will be shown below. Table 3 Thermodynamic parameters for boron adsorption onto the HAM. 3.5. Thermodynamic analysis T (K) ln K ΔG◦ (kJ⋅mol− 1) ΔH◦ (kJ⋅mol− 1) ΔS◦ (J⋅mol− 1⋅K− 1) 293 − 2.19 5.18 Thermodynamic parameters such as Gibbs free energy change (ΔG◦ ), 318 − 2.94 8.15 − 29.59 − 118.62 enthalpy change (ΔH◦ ), and entropy change (ΔS◦ ) can predict the 343 − 3.97 11.11 feasibility and nature of the adsorption process. ΔS◦ and ΔH◦ were Table 2 Parameters of different isotherms models of boron adsorption by HAM. The parameters are expressed in the following units: qm (mg⋅g− 1), KL (L⋅mg− 1), KF [(mg⋅g− 1) (L⋅mg− 1)1/n], AT (L⋅g− 1), bT (K⋅Jmol− 1). Langmuir isotherm Freundlich isotherm Temkin isotherm qm KL R2 KF 1/n R2 AT bT R2 138.50 0.0007 0.9949 0.25 0.80 0.9931 0.26 0.38 0.6993 4 X.-Y. Lou et al. Journal of Water Process Engineering 55 (2023) 104218 adding cations (Na+, Mg2+, K+, Ca2+, Cr2+, Fe2+, Ni2+, and Cu2+); or findings were also reported for boron adsorption on other metal oxide anions (Cl− , NO−3 , SO2− 3− 4 , PO4 ). The results for cations and anions are adsorbents [41,42]. The competitive adsorption of boron in simulated shown in Fig. 3a and b, respectively. tap water and ground water was also investigated. As depicted in Fig. 3d, The boron adsorption capacity was slightly lower in the presence of despite the presence of competitive anions and cations, the adsorption most of the cations. However, in the case of Cu2+, Cr3+, and Fe2+, the capacities of boron in tap water and ground water remained higher than adsorption capacity exhibited a significant increase. This notable 72.6 % and 80.5 %, respectively, compared to that in Milli-Q water. enhancement could be attributed to the formation of hydroxide pre­ cipitates when adjusting the pH of the solution up to 8.0, as shown in 3.7. Adsorption capacity comparison with similar adsorbent systems Fig. S5. These hydroxide precipitates can also adsorb boron ions. The results for the boron adsorption in the presence of the anions, As shown in Table 4, when comparing the maximum adsorption Fig. 3b, show that, for the molar ratio of [B]:[anion] = 1:1, the capacity of HAM with other adsorbents previously reported, we found adsorption capacity did not change noticeably and only a slight reduc­ that HAM performs better than most inorganic and organic adsorbent tion is observed respect to the case without having anions, therefore materials, and commercial resins. This outstanding adsorption is related these anions do not have a special affinity to HAM that could hamper the to the hollow dandelion-like porous structure of the HAM which facili­ adsorption of boron. To assess the effect of the concentration of the tates the interaction of the hydroxyl groups, typically present on the anions, the molar ratio [B]:[anion] was increased to 1:100. In this case, surface of nanostructured metal oxides, with the boron. Indeed, boric boron adsorption capacities were higher than those for 1:1. For most acid is known to have a strong affinity with polyols by chelation as re­ anions, the adsorption capacity was almost identical than the case ported by Bhagyaraj et al. [19]. without anions. However, for PO3− 4 , there was a noticeable increase in the adsorption capacity. This may be related to the compression of the electrical double layer on the HAM surface due to the presence of 3.8. Adsorption mechanism study trivalent phosphate which facilitates the interaction between the adsorption surface and the neutral boric acid species [40]. To corrobo­ The adsorption mechanism of boron on HAM was investigated using rate this, boron adsorption experiments were performed at different FTIR technique. The spectra of boric acid, HAM, and HAM after ionic strengths by adding NaCl (0–58.44 g⋅L− 1, see Fig. 3c). Initially, at adsorption with 200 mg⋅L− 1 and 1000 mg⋅L− 1 boron solutions are shown low ionic strength, below 2.92 g⋅L− 1, the adsorption capacity of boron in Fig. S6. It could be observed that the spectral profile of HAM did not decreased. This can be ascribed to the competing adsorption of Na+ and change after adsorption. This is mainly due to the fact that the amount of Cl− ions. When the ionic strength increased from 2.92 to 58.44 g⋅L− 1, boron adsorbed onto HAM is too small, lower than the detection limit of there is a concomitant increase in the boron adsorption. This increment the FTIR technique. can be attributed to the dominant effect of the double-layer compression The XPS technique was also utilized to determine the adsorption over the adsorption of Na+ and Cl− ions competing with B. In addition, mechanism. The spectra of HAM and HAM after adsorption with a 1000 this observation indicates that the mechanism of boron adsorption mg⋅L− 1 boron solution were analyzed, as shown in Fig. 4. Similar to the removal on HAM surface may be due to the formation of inner-sphere FTIR results, no significant new peaks appeared in the full-scan spectrum surface complexes rather than outer-sphere surface complexes. Similar after adsorption (see Fig. 4a). However, as shown in Fig. 4b, the pres­ ence of a B(1s) contribution confirms the successful adsorption of boron Fig. 3. Adsorption capacity of boron in the presence of competing cations (a), anions (b), different ionic strengths (c), and in simulated real waters (d). 5 X.-Y. Lou et al. Journal of Water Process Engineering 55 (2023) 104218 Table 4 3.9. Adsorption-desorption study Boron maximum adsorption capacities of several inorganic and organic adsor­ bents, and commercial resins. For practical applications, the regeneration capability of adsorbents Adsorbents Maximum adsorption Particle size Ref is a key parameter to assess. The adsorption capacity did not show sig­ capacity (mg⋅g− 1) (μm) nificant change after the 1st regeneration (see Fig. 5). From the 2nd to This 5th regeneration cycle, the adsorption capacity of HAM shows only a HAM 138.50 1–2 work slight decrease (<15.28 %) which demonstrates that this material has a Activated carbon 0.97 – [46] good regeneration capabilities regarding boron adsorption. Wood sawdust 1.58 – [46] Activated alumina 1.97 – [47] Composite 56.33 800–2300 [48] alginate–alumina Pyrocatechol-modified 19.45 1.3⋅10-3 [49] MCM-41 -3 MgO–Co-ferrite 0.25 43⋅10 [50] Li/Al-LDHs 42.91 – [51] NMDG modified 4-vinyl­ 7.19 28 ⋅10-3 [52] benzyl chloride NMDG@GPTMS-NBF 17.71 0.3 [53] CL-RESIN 8.37 722–855 [54] NCL-RESIN 8.57 710–845 [54] IRA743-RESIN 10.92 550–700 [54] P(GMA-co-TRIM)-EN-PG- 29.22 <241 [55] RESIN P(GMA-co-TRIM)-TETA- 23.25 <273 [55] PG-RESIN T-RESIN 21.25 – [56] on the HAM. Compared with the spectrum of boric acid, the binding energy of B(1s) in HAM appears at lower energy 192.61 eV, instead of 193.4 eV, after adsorption, indicating the formation of a bond. Furthermore, the binding energy at Al(2p) (see Fig. 4c) and O(1s) (see Fig. 4d) decreased after adsorption, which was attributed to the bonding of adsorbent with boron species at higher pH values than the point of zero charge [43]. All these results are consistence with previous studies [44,45] and strongly indicate the formation of Al-O-B bonds. Fig. 5. Adsorption capacity of boron after various adsorption/desorp­ tion cycles. Fig. 4. XPS spectra of HAM before and after adsorption. Full-scan spectrum (a), B 1s (b), Al 2p (c), and O 1s (d). 6 X.-Y. Lou et al. Journal of Water Process Engineering 55 (2023) 104218 4. Conclusions [5] F.S. Kot, Boron in the Environment, Boron Separation Processes 35, 2015, pp. 1–33, https://doi.org/10.1016/B978-0-444-63454-2.00001-0. [6] J.-Y. Lin, N.N.N. Mahasti, Y.-H. Huang, Recent advances in adsorption and In this study, HAM hollow spherical particles (~1.5 μm) with a coagulation for boron removal from wastewater: a comprehensive review, porous dandelion structure were synthesized through a microwave- J. Hazard. Mater. 407 (2021), 124401, https://doi.org/10.1016/j. assisted co-precipitation method and used for effective boron removal jhazmat.2020.124401. [7] J. Kluczka, T. 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