碳材於水圈汞污染控制之新穎應用

Novel Applications of Carbonaceous Materials for Aqueous Hg(II) Capture as Potential Methods for Pollution Control in Hydrosphere

許哲榮(Che-Jung Hsu)

指導教授 ： 席行正

國立臺灣大學/工學院/環境工程學研究所/博士(2021年)

https://doi.org/10.6342/NTU202103320

摘要

由於食物鏈及食物網的生物累積與生物放大，環境及工業中的汞分布對人體健康造成相當大的威脅，因此，汞的使用及排放為全球關注的重要議題。為了減緩汞排放對環境的衝擊，本論文合成官能化碳材並應用於工業汞控制（5.1、5.2及5.3節）與汞污染水體之環境整治（5.4節）。
於5.1節中，本論文藉由批次式吸附實驗獲取商用硫化活性碳（SAC）於石灰石濕式煙氣脫硫廢水中汞去除的最佳吸附參數。實驗結果顯示，在所有測試pH下，SAC1對液相汞（Hg(II)）的吸附量及去除率平均高於SAC2（CS2預處理之SAC1）0.32 mg/g與21%。此外，當pH由4上升至7，SAC1之Hg(II)吸附效果下降了22%（0.27 mg/g）。吸附動力擬合結果顯示擬二階與Elovich方程式可用以敘述SAC1對Hg(II)的吸附行為，由等溫吸附曲線擬合結果則可看出linear與Freundlich方程式較符合該化學吸附反應。吸附熱力學計算結果證實SAC1在該系統中對Hg(II)的吸附屬於自發性的放熱反應。於Hg0再逸散實驗結果可發現，當SO32-由0上升至0.01 mM，Hg0再逸散量下降了88%，SAC1的添加可完全抑制Hg0再逸散。整體而言，藉由添加SAC1可成功捕捉液相Hg(II)並抑制氣相Hg0再逸散發生。
5.2節則自行合成SAC，並於不同吸附參數下測試SAC在海水煙氣脫硫廢水中對Hg(II)之吸附行為。批次吸附結果顯示，當初始Hg(II)濃度高於4.7 µg/L，SAC之Hg(II)去除明顯優於AC。此外，SAC在pH 7及8下的吸附效果高於在pH 2至6之間。吸附動力擬合結果顯示擬二階方程式可用以敘述SAC對Hg(II)的吸附行為。由等溫吸附曲線擬合結果則可看出linear方程式較符合該化學吸附反應。吸附熱力學計算結果證實SAC在該系統中對Hg(II)的吸附屬於自發性的吸熱反應。另外，於Hg0再逸散實驗中發現NaClO的添加可有效降低Hg0再逸散，然而NaClO的添加促使Hg–Cl錯合物的形成，間接降低了SAC對Hg(II)的吸附。
合成銅硫共含浸活性碳（Cu-S-AC）並將其應用於海水煙氣脫硫廢水中的Hg(II)捕捉及Hg0再逸散抑制為5.3節的研究方向。批次吸附結果顯示，當初始Hg(II)濃度高於8 µg/L，Cu-S-AC之Hg(II)去除明顯優於硫含浸活性碳（S-AC）及AC。此外，Cu-S-AC在pH 7及8下的吸附效果高於在酸性條件。吸附動力擬合結果顯示擬二階方程式可用以敘述Cu-S-AC對Hg(II)的吸附行為，由等溫吸附曲線擬合結果則可看出線性與Freundlich方程式較符合該化學吸附反應。吸附熱力學計算結果證實Cu-S-AC在該系統中對Hg(II)的吸附屬於自發性的吸熱反應。Hg0再逸散實驗結果顯示，pH及溫度的上升會促進海水煙氣脫硫廢水中的Hg0再逸散，而Cu-S-AC的添加則可有效降低92%的Hg0再逸散。
將生物炭（biochar）的碳化、磁化及硫化合併為單一的熱處理程序以製備硫化磁性生物炭（SMBC）並將SMBC應用於水體Hg(II)去除為5.4節之研究主軸。批次吸附結果顯示，於600 °C下熱裂解合成的SMBC之Hg(II)最大吸附量（8.93 mg/g）高於400、500、700、800及900 °C下熱裂解合成的SMBC。此外，SMBC的Hg(II)吸附分別高於磁性生物炭（MBC）及一般生物炭（BC）53.0%及11.5%。酸性條件（pH 3.5–5）則有利於SMBC的Hg(II)吸附。吸附動力擬合結果顯示擬二階及外部質量傳輸方程式可用以敘述SMBC對Hg(II)的吸附行為。吸附熱力學計算結果證實SMBC在該系統中對Hg(II)的吸附屬於自發性的吸熱反應。SMBC在三種實際環境水體的吸附結果顯示，汞在淡水的分配係數（PC）為4.964 mg/g/µM，高於河口水（0.176 mg/g/µM）及海水（0.275 mg/g/µM），該數據也顯示鹽度對SMBC環境整治應用的影響。
總結而言，官能化AC（SAC及Cu-S-AC）可有效於濕式煙氣脫硫（石灰石或海水）廢水中捕捉液相Hg(II)並同時抑制氣相Hg0再逸散。除此之外，SMBC的合成快速且便捷，對Hg(II)吸附的效果良好，將來可實際應用於水體、底泥及土壤等環境介質之汞污染整治。本論文的研究成果不僅在污染控制技術或清潔生產有所貢獻，更針對工業排放及環境汞污染問題提出不同的新穎材料應用策略。綜上所述，本論文所產出之科學貢獻橫跨環境工程、材料科學及化學工程等三個重要領域。

關鍵字

汞 ； 再逸散 ； 活性碳 ； 生物炭 ； 吸附 ； 硫化 ； 磁化

並列摘要

Due to public health threats resulting from mercury (Hg) and its industrial/environmental distributions in the food chain/net, global restrictions have been placed on Hg use and releases. In this dissertation, functionalized carbonaceous adsorbents (i.e., activated carbon (AC) and biochar) are prepared and applied in scenarios representing industrial Hg control (Sections 5.1, 5.2, and 5.3) and environmental remediation of Hg-contaminated waters (Section 5.4).
In Section 5.1, a series of batch experiments were conducted to obtain the optimal adsorption condition for removing aqueous Hg(II) from actual Ca-added WFGD wastewater with commercial sulfurized activated carbon (SAC). The experimental results showed that SAC1 had an average 0.32 mg/g larger aqueous Hg(II) adsorption capacity and 21% larger Hg(II) removal than the CS2-treated SAC1 (i.e., SAC2) in all tested pH values. Furthermore, as increasing pH from 4 to 7, the Hg adsorption capacity of SAC1 decreased by 22% (i.e., 0.27 mg/g). Kinetic results showed that both pseudo-second-order and Elovich equations could well describe the chemisorption behavior of Hg(II) to SAC1. The equilibrium Hg(II) adsorption was well fitted with linear and Freundlich adsorption isotherms. Thermodynamic parameter calculation confirmed that Hg(II) adsorption by SAC1 was thermodynamically spontaneous and exothermic. Re-emission of gaseous Hg0 markedly decreased by 88% as SO32- addition increased from 0 to 0.01 mM. Notably, by the addition of SAC1, zero re-emission of gaseous Hg0 was achieved, confirming that the capture of aqueous Hg(II) and the inhibition of gaseous Hg0 re-emission can be successfully and simultaneously achieved via the addition of SAC into actual WFGD wastewater.
Section 5.2 investigated the dependence of Hg(II) adsorption behavior for synthesized SAC in actual SFGD wastewater on various influencing factors. SAC exhibited greater Hg(II) adsorption than AC at a C0 of more than 4.7 µg/L. The Hg(II) removal efficiency of SAC was slightly larger at pH 7 and 8 than that at pH within 2−6. Kinetic results showed that pseudo-second-order could well describe the chemisorption behavior of Hg(II) to SAC. Hg(II) adsorption on SAC was well correlated with the linear adsorption model. Thermodynamic analyses confirmed the endothermic and spontaneous adsorption behavior of Hg(II) on SAC in the seawater environment. Notably, the addition of NaClO significantly reduced the Hg(II) removal efficiency when SAC was used as the adsorbent. Nevertheless, NaClO addition also inhibited the reduction reaction of Hg(II) to Hg0 by forming strong Hg–Cl complexes.
In Section 5.3, the copper/sulfur co-impregnated activated carbon (Cu-S-AC) was synthesized to simultaneously capture aqueous Hg(II) and inhibit gaseous Hg0 re-emission from actual SFGD wastewater. Cu-S-AC exhibited greater Hg(II) adsorption than both AC and sulfur-impregnated activated carbon (S-AC) at a C0 of higher than 8 µg/L. The Hg(II) adsorption of Cu-S-AC was slightly greater at pH 7 and 8 than that under acidic conditions. The pseudo-second-order equation provided the best correlation coefficient for the Hg(II) adsorption on Cu-S-AC. The Hg(II) adsorption was well-fitted with both linear and Freundlich isotherms. In addition, the results of thermodynamic analyses veiled the endothermic and spontaneous adsorption of Hg(II) on Cu-S-AC. Notably, the increases of pH and temperature increased the Hg0 re-emission. Nevertheless, Cu-S-AC addition significantly inhibited the Hg0 re-emission (92%) from SFGD wastewater.
In Section 5.4, carbonization, magnetization, and sulfurization of biochar were combined into a single heating step to prepare sulfurized magnetic biochar (SMBC) for Hg(II) removal from water. Results indicate that SMBC prepared at 600 °C adsorbed 8.93 mg/g Hg(II), more than materials prepared at 400, 500, 700, 800, and 900 °C. Additionally, Hg(II) adsorption onto SMBC was 53.0% and 11.5% greater than onto magnetic biochar (MBC) and biochar (BC), respectively. Hg(II) adsorption is shown to be favorable in acidic conditions (pH 3.5–5), thermodynamically spontaneous, and endothermic. Adsorption results fit the pseudo-second-order and external mass transfer models. The partitioning coefficients (PC) were 4.964 mg/g/µM in freshwater, 0.176 mg/g/µM in estuary water, and 0.275 mg/g/µM in seawater, highlighting the importance of salinity in environmental remediation applications.
In summary, simultaneous capture of aqueous Hg(II) and inhibition of Hg0 re-emission from WFGD (Ca-added and/or SFGD) wastewaters was achieved by applying functionalized ACs (i.e., SAC and Cu-S-AC). Additionally, SMBC can be readily prepared with minimal processing steps, which is a robust adsorbent for Hg(II); it can potentially be applied to remediate contaminated water/sediment/soil in the future. The results of this dissertation contribute to the literature by not only describing individual control technologies and production strategies, but also introducing novel applications. The work contributes to Environmental Engineering, Materials Science and Engineering, and Chemistry.

並列關鍵字

mercury ； re-emission ； activated carbon ； biochar ； adsorption ； sulfurization ； magnetization

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