電化學輔助去除砷：整合陰極與陽極的法拉第反應

Abstract

地下水中的砷污染是全球亟待解決的問題，無論在發展中國家或已開發國家都造成了嚴重影響，故迫切需要高效且節能的除砷技術。然而，由於不帶電荷的三價砷親和力較低，使得現有技術在去除微量砷濃度與轉化三價砷為五價砷的過程中仍面臨諸多限制。針對這些挑戰，近年的除砷研究受電化學儲能技術的啟發，提出了新的解決策略，包括通過優化放電階段的電壓控制來促進三價砷的氧化效率，或開發具高砷親和力的電極材料來增強對三價砷的電催化活性。基於當前研究在電極材料與電化學程序的突破，我們進一步設計了新穎的電極材料，並提出創新的電化學輔助程序，旨在提高砷的去除與轉化效率。具體而言，我們開發了單金屬鈷氧化物與雙金屬鎳鈷氧化物修飾的奈米複合電極，並提出電輔助自鹼化及無氧化劑程序，為砷去除提供了更具創新性和有效性的解決方案。 在第一部分，我們通過將鈷氧化物奈米顆粒修飾在活性碳電極上，開發了雙功能鈷氧化物/活性碳奈米複合電極，成功實現三價砷的原位電催化氧化以及五價砷的高效電吸附。此鈷氧化物/活性碳電極具有540.2 m2 g−1的高比表面積，且展現出良好的導電性與電催化活性。應用在電容去離子技術中，三價砷的去除效率隨著pH上升而提高。特別是在pH 10的條件下，其電吸附容量達到0.75 mg g−1，且能源消耗低至0.12 kWh m−3。在充電/放電循環期間，三價砷氧化為五價砷的轉化效率達67%，生成的五價砷則可通過電吸附機制去除，從而大幅提升總砷的去除效率。此外，通過十次循環操作的實驗驗證，奈米複合電極展現優異的持久與再生性能，突顯其在實際應用中處理含砷地下水的可行性。 在第二部分，我們設計了雙金屬鎳鈷氧化物/活性碳奈米複合電極，通過電沉積方法將鎳和鈷氧化物修飾在活性碳上，以增強電化學除砷效率。由電化學特性分析的結果顯示，鎳鈷氧化物/活性碳電極中的鎳與鈷氧化物具備協同效應，能改善電子傳輸效率，並增強三價砷的電催化活性。應用在電容去離子技術中，施加1.2 V及pH 8的初始條件下，電吸附容量達到0.73 mg g−1，能源消耗僅為0.069 kWh m−3，表現明顯優於單金屬氧化物電極。三價砷的轉化效率也受到pH影響，在pH 10時，最高達75%的三價砷可氧化為五價砷。此外，由模擬地下水中進行多次循環實驗證實了該電極的穩定性與再生能力，顯示其在長期除砷應用中的潛力。 在第三部分，我們提出了電輔助自鹼化及無氧化劑程序，該程序實現了90.3%的砷去除效率，將陰極室內的三價砷濃度從150 µg L−1降至低於5 µg L−1。三價砷在陰極室經由鹼化解離成砷含氧陰離子，同時經由原位生成的過氧化氫氧化為五價砷。估計近80%的三價砷遷移至陽極室係歸因於過氧化氫的氧化，約20%則歸因於pH提升帶來的鹼化作用。在1.2至1.5 V的電壓條件下，過氧化氫的最高累積濃度達到10.9 mg L−1，從而在pH 4至10的寬廣範圍內增強砷的去除效率，最終實現高達97.0%的三價砷氧化為五價砷的轉化效率，且能源消耗最低僅為0.013 kWh m−3。此外，砷透過原位電氧化和電吸附作用被穩定地固定在陽極上，這將有助於後續廢棄物的處置。 本論文的研究成果強調了電極材料的精進與電化學程序的改良在砷污染地下水整治中的潛力與原創性，為環境工程領域貢獻了具實際應用價值的高效且低耗能解決方案。

Arsenic contamination in groundwater is a growing threat at the global level, emphasizing the urgent need for advanced, high-efficiency, low-energy remediation alternatives. With regards this, recent advances in arsenic remediation techniques, which involve ideas from electrochemical energy storage systems, have emerged responding to the problems of trace arsenic removal and detoxification. They encompass designing of electrode materials with increased arsenic affinity to improve As(III) electrocatalytic activity and controlling the voltage on discharge in improving As(III) oxidation. Building on these advancements in both electrochemical processes and electrode materials, this study presents novel electrode designs and introduces an innovative electrochemically-assisted process aimed at improving arsenic removal and conversion efficiency. Specifically, the research focuses on nanocomposite electrodes modified with monometallic cobalt oxide and bimetallic nickel-cobalt oxide, as well as the development of an electro-assisted self-alkalization and oxidant-free process (ESOP), providing a more effective and innovative solution for arsenic remediation. In the first part, we present a novel approach to incorporate cobalt oxide (CoOx) nanoparticles with activated carbon (AC), referred to as a bifunctional nanocomposite CoOx/AC electrode, and thereby achieve the simultaneous in-situ electrocatalytic oxidation of As(III) and the efficient electrosorption of As(V). The electrochemical measurements of the CoOx/AC electrode, which had a high specific surface area of 540.2 m2 g−1, demonstrated good electrical conductivity and electrocatalytic activity toward the As(III) oxidation reaction. Asymmetric CDI experiments with the CoOx/AC electrodes were performed at 1.2 V in batch-mode for different pH values. It is indicated that elevated pH can enhance the As(III) removal efficiency. Compared to the AC electrode, the CoOx/AC electrode had a considerably higher electrosorption capacity of 0.75 mg g−1 with a low energy consumption of 0.12 kWh m−3 at pH 10. To obtain insight into the mechanisms, the As(III)/As(V) concentrations and distribution were investigated in a charging-discharging cycle. When the CoOx/AC electrode was used as the anode, the electrocatalytic conversion of As(III) into As(V) was significantly enhanced from 43% to 67%, and then the generated As(V) could be electroadsorbed by electrical double-layer charging, thereby achieving an improvement in As(III) removal. Finally, the single-pass asymmetric CDI operated at ten consecutive charging-discharging cycles further demonstrated the great feasibility of using the CoOx/AC electrode to promote the in-situ electrocatalytic oxidation of As(III) for remediation of arsenic-contaminated groundwater. In the second part, we engineered the Ni1Co1/AC nanocomposite electrode by electrodepositing nickel and cobalt oxides onto activated carbon (AC) to enhance the electrochemical removal of arsenic. Electrochemical measurements of the Ni1Co1/AC electrode, which contained Ni(OH)2/NiOOH and Co3O4, demonstrated that nickel and cobalt oxides showed synergistic interactions in enhancing electron transport and the As(III) electrocatalytic activity. Meanwhile, asymmetric CDI experiments at 1.2 V in the batch mode using Ni1Co1/AC electrode have shown an excellent As removal capacity of 0.73 mg g−1 with low energy consumption of 0.069 kWh m−1 at pH 8, way superior to that obtained on monometallic electrodes of Ni2/AC and Co2/AC. The capacity of As(III) removal increased with increasing pH; at pH 10, a maximum efficiency in the conversion of As(III) to As(V) was realized at 75%, which suggested effective in-situ electrocatalytic oxidation. Mechanistic results also indicated that the nickel and cobalt oxides were involved in the in-situ electrocatalytic oxidation of As(III) into As(V), thus improving the overall efficiency of arsenic removal. Long-term stability and regeneration of the electrode were investigated in simulated groundwater in a single-pass mode. It is evident from these findings that the bimetallic Ni-Co oxides modified AC electrode can be quite powerful and efficient in the removal of arsenic from contaminated groundwaters. In the third part, an electro-assisted self-alkalization and oxidant-free processes cell was developed and investigated. It was found that the ESOP removed 90.3% of arsenic and reduced the As(III) concentration from 150 µg L−1 to less than 5 µg L−1 in its cathode chamber. The As removal involved migration of As(III) and As(V) from the cathode to the anode driven by electrical current. In the ESOP cathode, As(III) was dissociated to As(III) oxyanions via alkalization and then oxidized into As(V) by H2O2. Nearly 80% of As(III) migration could be attributed to the oxidation by H2O2 and approximately 20% dissociation by pH alkalization. The voltage-controlled conditions (1.2−1.5 V) achieved a peak cumulative H2O2 concentration of 10.9 mg L−1. The ESOP demonstrated a high As(III) oxidation to As(V) conversion efficiency of 97.0% as well as a low energy cost of 0.013 kWh m−3 at 1.2 V. The migrated arsenic was stabilized onto the anode electrode through in-situ electro-oxidation of As(III) and electrosorption of As(III, V); this would help with the post-treatment waste disposal. Those results have provided important insights into an electrochemical approach for highly efficient arsenic detoxification. As per these findings, there exists the high potential to sustainably depollute arsenic contamination in groundwater within a reasonable energy budget thereby enhancing their usability and offering valuable perspectives in environmental engineering.