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WANG Lei, LI Luyao, YANG Zhenglong, et al. Microstructure of calcium-iron modified biochar and its effect on phosphate adsorption from aqueous solution[J]. Journal of Zhejiang A&F University, 2025, 42(X): 1−11 doi:  10.11833/j.issn.2095-0756.20250128
Citation: WANG Lei, LI Luyao, YANG Zhenglong, et al. Microstructure of calcium-iron modified biochar and its effect on phosphate adsorption from aqueous solution[J]. Journal of Zhejiang A&F University, 2025, 42(X): 1−11 doi:  10.11833/j.issn.2095-0756.20250128
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Microstructure of calcium-iron modified biochar and its effect on phosphate adsorption from aqueous solution

DOI: 10.11833/j.issn.2095-0756.20250128
  • 1.

    School of Environment and Natural Resources, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China

  • 2.

    College of Environment and Resources, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China

  • 3.

    Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China

  • Received Date: 2025-01-26
  • Accepted Date: 2025-06-17
  • Rev Recd Date: 2025-06-02
    • Available Online: 2025-09-08
  •   Objective  This study investigates the influence of pyrolysis temperature and calcium-iron (Ca/Fe) modification on the pore structure of biochar and its phosphate adsorption performance, aiming to provide reference for the application of modified biochar in the field of water treatment.   Method  Biochars and Ca/Fe-modified biochars were synthesised at 300 , 500 , and 700 ℃, and characterised using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) to investigate their microstructure and surface chemistry. Batch adsorption experiments were conducted to evaluate the effects of pyrolysis temperature, dosage, solution pH, and co-existing ions on phosphate removal. Furthermore, adsorption mechanisms were elucidated using isotherm and kinetic models.   Result  With increasing pyrolysis temperature, the pore structure of biochar developed progressively. Notably, Ca/Fe-modified biochar produced at 700 ℃ exhibited a highly ordered porous architecture and enlarged specific surface area, achieving a maximum phosphate adsorption capacity of 890.836 mg·g−1 and a removal efficiency of 90.32%. Adsorption isotherms fitted the Sips model, while kinetic data followed the pseudo-second-order model, indicating chemisorption as the dominant mechanism. XRD analysis confirmed the formation of Ca(OH)2 at high temperature, which contributed to enhanced phosphate fixation. Adsorption performance was influenced by pH, dosage, and co-existing ions; alkaline conditions and optimised dosage improved phosphate removal, whereas competing anions such as ${\mathrm{NO}}_3^- $ and $ {\mathrm{SO}}_4^{2 - } $ inhibited the adsorption process.   Conclusion  Tuning pyrolysis temperature and Ca/Fe modification significantly enhances the pore architecture and adsorption performance of biochar, offering an efficient material for phosphate removal from aqueous environments. [Ch, 5 fig. 4 tab. 32 ref.]
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Microstructure of calcium-iron modified biochar and its effect on phosphate adsorption from aqueous solution

doi: 10.11833/j.issn.2095-0756.20250128
  • 1. School of Environment and Natural Resources, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China
  • 2. College of Environment and Resources, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China
  • 3. Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, Zhejiang University of Science and Technology, Hangzhou 310023, Zhejiang, China
  • Received Date: 2025-01-26
  • Accepted Date: 2025-06-17
  • Rev Recd Date: 2025-06-02
  • Available Online: 2025-09-08

Abstract:   Objective  This study investigates the influence of pyrolysis temperature and calcium-iron (Ca/Fe) modification on the pore structure of biochar and its phosphate adsorption performance, aiming to provide reference for the application of modified biochar in the field of water treatment.   Method  Biochars and Ca/Fe-modified biochars were synthesised at 300 , 500 , and 700 ℃, and characterised using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) to investigate their microstructure and surface chemistry. Batch adsorption experiments were conducted to evaluate the effects of pyrolysis temperature, dosage, solution pH, and co-existing ions on phosphate removal. Furthermore, adsorption mechanisms were elucidated using isotherm and kinetic models.   Result  With increasing pyrolysis temperature, the pore structure of biochar developed progressively. Notably, Ca/Fe-modified biochar produced at 700 ℃ exhibited a highly ordered porous architecture and enlarged specific surface area, achieving a maximum phosphate adsorption capacity of 890.836 mg·g−1 and a removal efficiency of 90.32%. Adsorption isotherms fitted the Sips model, while kinetic data followed the pseudo-second-order model, indicating chemisorption as the dominant mechanism. XRD analysis confirmed the formation of Ca(OH)2 at high temperature, which contributed to enhanced phosphate fixation. Adsorption performance was influenced by pH, dosage, and co-existing ions; alkaline conditions and optimised dosage improved phosphate removal, whereas competing anions such as ${\mathrm{NO}}_3^- $ and $ {\mathrm{SO}}_4^{2 - } $ inhibited the adsorption process.   Conclusion  Tuning pyrolysis temperature and Ca/Fe modification significantly enhances the pore architecture and adsorption performance of biochar, offering an efficient material for phosphate removal from aqueous environments. [Ch, 5 fig. 4 tab. 32 ref.]

WANG Lei, LI Luyao, YANG Zhenglong, et al. Microstructure of calcium-iron modified biochar and its effect on phosphate adsorption from aqueous solution[J]. Journal of Zhejiang A&F University, 2025, 42(X): 1−11 doi:  10.11833/j.issn.2095-0756.20250128
Citation: WANG Lei, LI Luyao, YANG Zhenglong, et al. Microstructure of calcium-iron modified biochar and its effect on phosphate adsorption from aqueous solution[J]. Journal of Zhejiang A&F University, 2025, 42(X): 1−11 doi:  10.11833/j.issn.2095-0756.20250128
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  • 人类活动引发的磷(P)输入已成为生态系统的重要干扰因素，尤其在工业废水和生活污水排放中，磷的过量积累显著加剧了水体富营养化问题[1]。在多种磷去除技术中，吸附法因简便、高效和广泛适用性而备受关注[2]。理想的吸附剂应具备低成本、环境友好、高吸附容量及优良可再生性等特点。生物质炭作为一种绿色吸附材料，因其高含碳、大孔隙率、高比表面积以及丰富的官能团，展现出优越的吸附性能[3−4]。然而，由于生物质炭表面多为负电荷，对阴离子特别是磷酸根的吸附能力较弱(最大吸附量平均为28.9 mg·g−1)，限制了生物质炭的应用潜力[5]。对生物质炭进行改性，增强表面活性位点、优化孔隙结构和提高阳离子交换容量，从而提升生物质炭对磷酸根的化学结合能力和吸附性能[6]，是当前吸附材料领域新的研究方向。

    金属改性凭借其多样化的吸附机制和广泛的适用性，成为提升生物质炭吸附性能的重要手段[7]。例如，铝改性通过铝离子(Al3+)与磷酸根的络合作用显著提高吸附容量[8]；铁改性利用铁离子(Fe3+)与磷酸根的强配位作用实现高效吸附[9−10]；铁锰复合改性依靠氧化还原特性表现出协同吸附优势，适用于去除多种污染物[11]；钙镁改性通过与磷酸根形成稳定沉淀，在高pH条件下展现出更优的吸附能力[12]。牡蛎Ostrea edulis壳制备的钙改性生物质炭吸附容量可达96.0 mg·g−1[13]。经过镁改性处理后的花生Arachis hypogaea壳生物质炭的吸附性能得到显著提升，最大吸附容量达44.1 mg·g−1，较未改性前提高了60%[14]。尽管如此，上述改性方法在吸附速率和吸附量方面仍存在优化空间，因此开发兼具更高吸附容量、更优稳定性及经济可行性的改性技术已成为研究的重点方向。

    与以往金属改性方式相比，钙铁(Ca/Fe)复合改性具有独特优势：①钙离子与磷酸根的亲和力强，能够快速形成稳定的Ca-P沉淀，提高吸附效率[15]；②铁离子不仅能提供额外的活性位点，还具有较高的阳离子交换容量，有助于在复杂水体环境中保持吸附稳定性[16]；③钙铁资源丰富、价格低廉，具备较高的经济可行性和工业应用潜力[17]。然而，钙铁改性生物质炭对磷酸根的吸附性能及吸附机制尚不明确。因此，本研究以玉米Zea mays秸秆为原料，制备钙铁改性生物质炭(Ca/Fe-CSB)，通过多种表征手段及吸附动力学和热力学分析，系统研究改性方式及热解温度对生物质炭物理化学特性、磷吸附性能的影响，以期为水体磷污染治理和磷资源回收提供理论支撑与实践指导。

    • 1.   材料与方法

      1.1.   吸附剂的制备

      1.1.1.   生物质炭的制备

    • 使用废弃玉米秸秆制备生物质炭。玉米秸秆用高速粉碎机粉碎，过20目筛。将10 g粉状秸秆装入管式炉中，以10 ℃·min−1的升温速率，分别在300、500和700 ℃的恒温下热解2 h。待管式炉冷却至室温后取出，研磨、过100目筛，制得生物质炭粉末，分别命名为CSB-300、CSB-500和CSB-700。

      • 1.1.2.   改性生物质炭制备

    • 将10 g粉状秸秆置于100 mL浓度为0.20 mol·L−1的六水合三氯化铁(FeCl3·6H2O)的溶液中，室温下超声30 min，然后加入0.04 mol·L−1氢氧化钙[Ca(OH)2]，在磁力搅拌器上60 ℃老化2 h，离心收集固相部分，得到负载Ca/Fe的原料。将Ca/Fe原料放入管式炉中300、500和700 ℃热解炭化2 h，待管式炉冷却至室温后取出，研磨、过100目筛，制得生物质炭粉末，得到Ca/Fe改性生物质炭分别命名为Ca/Fe-CSB-300、Ca/Fe-CSB-500和Ca/Fe-CSB-700。

      • 1.2.   生物质炭表征

    • 通过扫描电子显微镜(SEM)分析材料的微观形态，利用X射线衍射仪(XRD)测定晶型结构，使用表面特征分析仪(BET)测定材料的比表面积和孔隙结构，采用傅里叶红外光谱仪(FTIR)分析生物质炭的结构和化学组成。

      • 1.3.   生物质炭吸附实验

      1.3.1.   生物质炭制备温度对吸附的影响

    • 取40 mL磷酸盐溶液(50 mg·L−1)于150 mL锥形瓶中，分别加入0.04 g生物质炭(CSB-300、CSB-500、CSB-700、Ca/Fe-CSB-300、Ca/Fe-CSB-500和Ca/Fe-CSB-700)，然后放置于恒温振荡器中，在25 ℃，200 r·min−1条件下振荡12 h达到吸附平衡。结束后用0.45 μm滤膜过滤，采用钼锑抗分光光度法测定磷酸盐质量浓度，按以下公式计算吸附量。

      式(1)中：qt为t时生物质炭对磷的吸附量(mg·g−1)；C0和Ct分别为初始时间和t时磷的质量浓度(mg·L−1)；V为溶液体积(L)；m为生物质炭的投加量(g)。

      • 1.3.2.   生物质炭投加量对吸附的影响

    • 分别称取0.04、0.06、0.08、0.10、0.15、0.18、0.20 g生物质炭(Ca/Fe-CSB-700)于50 mL离心管，加入40 mL磷酸盐溶液(50 mg·L−1)，投加量分别为1.00、1.50、2.00、2.50、3.75、4.50、5.00 g·L−1，其余步骤同1.3.1。

      • 1.3.3.   溶液pH对吸附的影响

    • 调节磷酸盐溶液pH为2~12，取40 mL调制后的磷酸盐溶液于50 mL离心管，加入0.04 g生物质炭(Ca/Fe-CSB-700)，其余步骤同1.3.1。

      • 1.3.4.   共存离子对吸附的影响

    • 在pH 8的磷酸盐溶液(50 mg·L−1)中，分别加入0.001 mol·L−1的硫酸根(${\mathrm{SO}}_4^{2 - } $)、碳酸氢根(${\mathrm{HCO}}_3^{ - } $)、硝酸根(${\mathrm{NO}}_3^{ - } $)和氯离子(Cl−)溶液，混合均匀后，取40 mL溶液置于离心管中，加入0.04 g生物质炭(Ca/Fe-CSB-700)。其余步骤同1.3.1。

      • 1.4.   吸附动力学

    • 分别称取0.04 g Ca/Fe-700和Ca/Fe-CSB-700于50 mL离心管中，加入40 mL质量浓度为50 mg·L−1的磷酸盐溶液，在25 ℃、200 r·min−1下搅拌。分别在5、15、30、60、90、180、360、540、720和1 440 min取样，测定磷酸盐质量浓度并计算吸附量[式(1)]。采用准一级动力学和准二级动力学模型拟合试验数据。模型等式为：

      式(2)~(3)中：qt表示t时吸附剂上的磷酸盐吸附量(mg·g−1)；qe为平衡时吸附量(mg·g−1)；t为吸附时间(min)；k1为准一级动力学速率常数(min−1)；k2为准二级动力学速率常数(g·mg−1·min−1)。

      • 1.5.   吸附热力学

    • 分别称取0.04 g CSB-700和Ca/Fe-CSB-700于50 mL离心管中，加入40 mL质量浓度分别为5、10、30、50、70、90、120 mg·L−1的磷酸盐溶液中，于25 ℃恒温振荡12 h。结束后测定磷酸盐质量浓度。根据以下公式计算平衡吸附量：

      式(4)中：qe为吸附平衡时的吸附量(mg·g−1)；C0为初始时间质量浓度(mg·L−1)；Cₑ为吸附平衡时的质量浓度(mg·L−1)；V为溶液体积(L)；m为吸附剂质量(g)。

      将吸附数据用Langmuir、Freundlich和Sips模型拟合。模型等式分别为：

      式(5)~(7)中：qe为吸附平衡时的吸附量(mg·g−1)；qm为最大吸附量(mg·g−1)；KL为Langmuir吸附平衡常数(L·mg−1)；Ce为吸附平衡时的质量浓度(mg·L−1)；KF为Freundlich常数(mg1−1/n·g−1·L1/n)；1/n为非线性参数；K为Sips模型的平衡常数(L·mg−1)；n为指数参数，反映系统的非均匀性。

      热力学参数的计算公式为：

      式(8)~(10)中：G0为吉布斯自由能(J·mol−1·K−1)；S0为标准熵(J·mol−1·K−1)；H0为标准焓；Ke为热力学平衡常数(L·mol−1)；R为理想气体常数，取值为8.314 J·mol−1·K−1；T为热力学温度(K)。

      • 1.6.   数据处理

    • 采用Excel处理数据，采用SPSS 26进行方差分析和差异显著性检验，使用Origin 2021绘图。每个实验重复3次。

    2.   结果与分析

      2.1.   生物质炭的表征

    • 如表1和图1A所示：随着热解温度从300 ℃升至700 ℃，未改性和Ca/Fe改性生物质炭逐步形成稳定且规则的孔隙结构。与CSB-700相比，Ca/Fe-CSB-700表现出高度有序的孔隙结构，形成了密集的规则孔隙排列，且孔隙大小均匀。

      生物质炭 元素质量百分比/% 比表面积/(m2·g−1) 平均孔径/nm
      C O Ca Fe
      CSB-300 68.33±1.12 c 23.55±1.60 b 0 0 2.87±0.06 e 5.49±0.07 b
      Ca/Fe-CSB-300 60.89±1.56 d 30.63±0.60 a 2.59±0.21 c 1.20±0.05 c 5.54±0.25 e 8.22±0.87 a
      CSB-500 80.44±2.50 b 12.75±1.54 c 0 0 31.27±3.85 d 2.55±0.77 d
      Ca/Fe-CSB-500 61.41±1.40 d 13.29±2.66 c 6.40±0.21 b 5.57±0.39 a 81.80±2.13 c 3.61±0.10 c
      CSB-700 85.02±2.64 a 8.66±1.39 d 0 0 192.58±4.34 b 2.27±0.13 d
      Ca/Fe-CSB-700 55.15±2.06 e 32.80±2.90 a 6.67±0.18 a 4.98±0.44 b 218.94±3.04 a 2.95±0.24 cd
      　　说明：不同小写字母表示同一指标不同处理间差异显著(P＜0.05)。

      Table 1.  Elemental composition, specific surface area, and pore size analysis of biochar before and after modification

      Figure 1.  SEM images (A), XRD patterns (B), and FTIR spectra (C) of raw and modified biochar at different pyrolysis temperatures

      XRD分析结果(图1B)显示：Ca/Fe改性生物质炭均检测到Ca(OH)2晶相，这些晶相的形成提供了额外的离子交换位点。Ca/Fe-CSB-700表现出较低的结晶度，主要呈现无定形碳结构，而晶相则更加有序，结晶度明显提高。

      FTIR分析结果(图1C)显示：生物质炭在3 444 cm−1处对应—OH伸缩振动，且随热解温度的升高逐渐减弱，反映水分和挥发性有机物的脱除过程。300 ℃下，1 606 cm−1的特征峰指示C=O和C=C振动，表明羰基及碳-碳双键的存在；1 025 cm−1峰处与C—O—C伸缩振动相关，说明存在醚键结构；466 cm−1峰则对应Si—O—Si弯曲振动，表明硅酸盐残留。在500和700 ℃下，羰基和C—O—C特征峰虽仍可见，但强度逐渐减弱，指示有机结构热解的进一步加深。

      • 2.2.   最佳热解温度和投加量的确定

    • 与未改性生物质炭相比，Ca/Fe改性显著提高了生物质炭的除磷性能(P＜0.05)，并且在500和700 ℃条件下表现出卓越的吸附能力(图2A)。在300 ℃时，Ca/Fe-CSB-300的去除率为61.77%，随着温度升高至700 ℃时，Ca/Fe-CSB-700的去除率达90.32%。此外，固液比的变化显著影响了吸附效率，随着投加量的增加，磷酸盐去除率逐渐上升，并在3.75 g·L−1时达到稳定(图2B)，表明此时的投加量实现了吸附位点的最大化利用。因700 ℃ 条件下，1.00 g·L−1的Ca/Fe改性生物质炭(Ca/Fe-CSB-700)对磷酸盐的去除率已经很高(图2A)，故本研究选择700 ℃热解温度、1.00 g·L−1固液比对改性前后生物质炭的吸附性能和机制进行进一步探究。

      Figure 2.  Phosphate removal efficiency at different pyrolysis temperatures (A) and dosage (B)

      • 2.3.   Ca/Fe改性生物炭吸附机制探究

      2.3.1.   吸附动力学和热力学

    • 通过吸附动力学模型对数据进行拟合，结果显示：准二级动力学模型(决定系数R²＞0.93)的拟合度高于准一级动力学模型(图3和表2)，表明Ca/Fe-CSB-700和CSB-700的吸附过程主要受化学吸附机制控制。根据准二级动力学模型的拟合结果，Ca/Fe-CSB-700的最大吸附量(qm)从102.43 mg·g−1提高至917.39 mg·g−1(表2)，增幅达8.96倍。此外，Ca/Fe-CSB-700的拟合曲线在初始阶段增长速度明显高于CSB-700，表明改性材料加速了化学吸附过程。

      Figure 3.  Adsorption kinetics and isotherm model fitting of CSB-700 (A－D) and Ca/Fe-CSB-700 (E－H)

      生物质炭 准一级动力学方程 准二级动力学方程
      qm k1 R2 qm k2 R2
      CSB-700 95.44 0.01 0.90 102.43 0.00 0.93
      Ca/Fe-CSB-700 871.87 0.02 0.91 917.39 0.05 0.96

      Table 2.  Adsorption kinetic fitting parameters of CSB-700 and Ca/Fe-CSB-700

      通过等温吸附模型Langmuir、Freundlich和Sips对生物质炭吸附数据进行拟合发现：Sips模型拟合度最好(图3和表3)。Langmuir模型显示：Ca/Fe-CSB-700在298 K下的最大吸附量(qm)是CSB-700的3.52倍(表3)，表明改性明显增加了有效吸附位点的数量与活性。Freundlich模型显示：Ca/Fe-CSB-700的吸附常数和非线性参数(1/n)显著提升(表3)。Sips模型拟合结果显示：Ca/Fe-CSB-700的最大吸附量(qm)达890.836 mg·g−1，是CSB-700的4.7倍。

      生物质炭 T(K) Langmuir模型 Freundlich模型 Sips模型
      qm/
      (mg·g−1)      		 KL/
      (L·g−1)      		 R2 KF/
      (mg1−1/n·g−1·L1/n)      		 1/n R2 qm/
      (mg·g−1)      		 K/
      (      L·mg−1)      		 n R2
      CSB-700 288 152.91 0.08 0.92 19.15 0.50 0.96 143.41 0.01 0.55 0.95
      298 166.39 0.02 0.93 1.88 1.15 0.93 189.81 0.02 1.41 0.92
      308 298.51 0.02 0.97 6.60 0.82 0.97 199.14 0.01 1.01 0.96
      Ca/Fe-CSB-700 288 324.13 0.07 0.90 38.16 0.75 0.96 828.39 0.07 2.65 0.98
      298 585.82 0.04 0.92 34.99 0.79 0.98 890.84 0.06 2.21 0.99
      308 232.78 0.11 0.91 46.76 0.72 0.96 900.71 0.07 2.38 0.98

      Table 3.  Adsorption kinetic fitting parameters of CSB-700 and Ca/Fe-CSB-700

      如表4所示：与CSB-700相比，Ca/Fe-CSB-700在298 K时的平衡常数(Ke)从3.23 L·mol−1提高至15.58 L·mol−1，表明改性明显增强了吸附能力。吉布斯自由能(ΔG0)的降低(−6.35至−6.98 kJ·mol−1)显示出吸附过程的自发性。较小焓变(ΔH0=1.59 kJ·mol−1)表明吸附过程对温度变化的敏感性较弱，具有较好的环境适应性。熵变(ΔS0)增加(28.14 J·mol−1·K−1)反映出系统混乱度增加，可能与吸附位点增多及磷酸根与活性位点之间更强的络合作用有关。

      生物质炭 T(K) Ke/(L·mol−1) ΔG0/(kJ·mol−1) ΔH0/(kJ·mol−1) ΔS0/(J·mol−1·K−1)
      CSB-700 288 3.09 −2.70 2.95 19.64
      298 3.23 −2.90
      308 3.35 −3.09
      Ca/Fe-CSB-700 288 14.17 −6.35 1.59 28.14
      298 15.58 −6.80
      308 15.25 −6.98

      Table 4.  Thermodynamic parameters for Ca/Fe-CSB-700

      • 2.3.2.   生物质炭吸附反应前后官能团对比

    • 由图4可知：与CSB-700相比，Ca/Fe-CSB-700在吸附后的1 060 cm−1处出现了P—O特征峰，表明磷酸根成功结合至材料表面。同时Ca/Fe-CSB-700表面富含—OH (3 444 cm−1)和C=O (1 606 cm−1)，表明Ca/Fe改性明显增强了表面官能团与磷酸根的化学作用，有效提高吸附性能。

      Figure 4.  FTIR spectra of CSB-700 and Ca/Fe-CSB-700 before and after adsorption

      • 2.3.3.   不同pH和阴离子对Ca/Fe改性生物质炭吸附性能的影响

    • 由图5A可见：初始溶液pH对吸附性能具有显著影响(P＜0.05)。总体来说，磷酸盐吸附量随着pH的升高而增加，且在pH≥8时显著增加，在pH为11时达到最大值。Zeta电位分析显示：Ca/Fe-CSB-700的0点电位pH (pHpzc)为2.83 (图5B)。当溶液的pH低于pHpzc时，吸附剂表面带正电，pH高于pHpzc时，吸附剂表面带负电。随着pH升高，Ca/Fe-CSB-700表面Zeta电位逐渐降低，负电荷增加，从而增强了对${\mathrm{HPO}}_4^{2 - } $的静电吸附作用[18]。共存阴离子对磷酸盐吸附表现出不同程度的抑制作用(图5C)，其中Cl−影响最小，${\mathrm{HCO}}_3^{ - } $居中，${\mathrm{NO}}_3^{ - } $和${\mathrm{SO}}_4^{2 - } $的抑制效应最为显著。

      Figure 5.  Effect of pH (A, B) and coexisting anions (C) on adsorption properties of Ca/Fe-CSB-700

    3.   讨论

      3.1.   热解温度和改性对生物质炭吸附性能的影响

    • 本研究表明：热解温度和改性处理通过影响生物质炭的孔隙结构和表面化学性质，显著增强了对磷酸根离子的吸附能力。磷酸盐去除实验与吸附等温线的拟合结果显示：与未经改性的生物质炭相比，Ca/Fe-CSB具有更优异的磷酸盐去除性能，这主要归因于Ca、Fe元素的引入，形成了更多的活性吸附位点，并提高了孔隙结构的异质性[19]。同时，Ca/Fe-CSB吸附能力随着热解温度的升高而增强，一方面是由于较高的热解温度(尤其是700℃)显著提高了孔隙的发育程度和比表面积，从而提升了吸附性能[20]；另一方面，热解过程促进了Ca(OH)2晶相的形成，提供额外的离子交换位点，进一步增强了对磷酸盐的吸附能力[21]。此外，随着热解温度的升高，Ca/Fe-CSB的晶相更加有序，显示出较高的结晶度，而这种稳定的结晶结构被认为是提升生物质炭吸附性能的关键因素[22]。

      • 3.2.   Ca/Fe改性生物质炭吸附性能的关键因素

    • 溶液pH和共存离子是影响Ca/Fe改性生物质炭吸附性能的关键因素[23]。本研究中，Ca/Fe-CSB-700在不同初始pH条件下的吸附行为表现出显著差异。随着pH从2升至12，磷酸盐的吸附量逐渐增大，并在pH为11时达到最大值。这一现象可能与pH变化对溶液中磷酸根离子形态的影响以及生物质炭表面电荷分布的变化有关[24]。在高pH条件下，磷酸根主要以${\mathrm{HPO}}_4^{2 - } $的形式存在，而Ca/Fe-CSB-700表面可能带有正电荷，从而增强了其对磷酸根的静电吸引作用[18]。这一结果与热力学分析中ΔG0负值的结果相符，表明高pH条件下吸附过程更具自发性。因此，调节溶液的初始pH可以有效优化吸附效率，特别是在高pH环境下，磷酸根的去除效果更为显著。

      此外，共存阴离子的存在对磷的吸附产生了明显的影响。本研究表明：不同阴离子对磷的吸附有不同程度的抑制作用，其中Cl−的影响最小，而${\mathrm{NO}}_3^{ - } $和${\mathrm{SO}}_4^{2 - } $的抑制效应较为显著，${\mathrm{HCO}}_3^{ - } $则介于两者之间。这可能与不同阴离子在溶液中竞争吸附位点以及静电排斥效应有关[25]。特别是${\mathrm{SO}}_4^{2 - } $和${\mathrm{NO}}_3^{- } $阴离子，它们可能与磷酸根离子竞争相同或相似的吸附位点，且由于竞争性较强，可能会导致对磷酸盐吸附有更显著的抑制效应[26]。因此，水体中共存阴离子的种类和浓度对Ca/Fe改性生物质炭的吸附性能具有重要影响，这一现象在实际应用中需要加以考虑，以便优化吸附效果。

      • 3.3.   吸附机制分析

    • Ca/Fe-CSB-700在磷酸盐吸附中的表现可以归因于多种机制的协同作用。FTIR和XRD分析表明：未吸附磷酸盐的Ca/Fe-CSB-700表面具有丰富的羟基、羰基和芳香族化合物等负电荷官能团，这些官能团为磷酸根的吸附提供了基础[27]。吸附后FTIR图谱中出现了磷酸根特征的P—O吸收峰，表明磷酸根离子与生物质炭表面的官能团发生了明显的相互作用，尤其是在碱性条件下，磷酸根主要以${\mathrm{HPO}}_4^{2 - } $形式存在，与表面正电荷产生强烈的静电吸引，从而增强了磷酸盐的吸附[28]。此外，Ca/Fe-CSB-700表面丰富的金属离子Ca2+和Fe3+为磷酸根提供了离子交换位点，并通过离子交换或络合作用固定磷酸根离子。表面沉淀作用加上XRD结果显示：Ca(OH)2晶相在热解过程中形成，这不仅为磷酸根提供了更多的离子交换位点，还促进了磷酸根的吸附。Ca(OH)2的形成能够增强磷酸根的固定吸附能力，这与磷酸盐吸附实验中的高去除率一致。

      化学吸附作用也在磷酸盐吸附过程中发挥了重要作用。FTIR分析表明：吸附后生物质炭表面的官能团发生了显著变化[29]，可能通过氢键和金属-磷络合作用进一步稳定磷酸根离子的吸附[30]。特别是在高pH条件下，磷酸根离子通过化学键与表面官能团结合，进一步增强了磷酸盐的去除效率[31]。络合作用和表面沉淀作用也是磷酸盐吸附的重要机制[32]，Ca2+和Fe3+离子能够与磷酸根离子形成复合物，促进磷酸盐的沉淀并进一步提高其去除效率。

      综合来看，Ca/Fe-CSB-700对磷酸盐的吸附主要依赖于表面富集的羟基、羰基、Ca2+和Fe3+的静电吸附、离子交换、化学吸附、表面沉淀和络合作用的协同作用，使磷酸根离子在高效吸附过程中得到稳定固定。

    4.   结论

    • 本研究表明：随着热解温度升高，生物质炭的孔隙结构得到显著优化，特别是在700 ℃时，Ca/Fe-CSB展现出较大的比表面积和有序孔隙结构，最大吸附量达890.836 mg·g−1，明显优于未改性生物质炭。Ca/Fe改性不仅提高了生物质炭的孔隙稳定性，还引入了新的活性位点，如Ca2+和Fe3+，显著促进了磷酸根的吸附。磷酸根离子的吸附机制主要由静电吸引、离子交换、化学吸附和络合作用协同驱动。在碱性条件下，${\mathrm{HPO}}_4^{2 - } $离子的静电吸引作用显著增强，同时表面丰富的Ca2+和Fe3+通过离子交换与络合作用进一步提高了磷酸盐的吸附能力。综上，Ca/Fe-CSB-700通过多机制协同作用展现出优异的磷酸盐吸附性能，为水体磷污染治理提供了高效的功能材料与理论依据。

Reference (32)

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