醋酸及硫脲修飾法提升大麥渣對重金屬離子吸附效能之研究

Enhancement of Heavy Metal Ions Adsorption Capacity of Barley Waste Modified with Acetic Acid and Thiourea

電鍍、礦業及金屬加工處理業廢水中含有重金屬離子。水中金屬離子不能被生物分解且易累積蓄存在水體生物，經食物鏈進入人體中造成各種疾病（如痛痛病、水俣病等）。大麥渣(Hordeum spp.)經硫脲、硫脲+醋酸進行化學法修飾成吸附劑，確實可提升大麥渣去除水中重金屬離子Cu2+、Zn2+、及Pb2+能力，可同時處理食品加工廢棄物和含重金屬廢水二種環境污染問題。本研究針對pH值、接觸時間、溫度、重金屬離子種類對化學修飾化吸附劑吸附重金屬離子量之影響進行吸附特性探討。採用Freundlich equation、Langmuir equation、及Dubinin-Radushkevich equation三種等溫吸附平衡模式，Pseudo-first order equation 、Pseudo-second order equation 、Intraparticle diffusion equation三種吸附動力模式及計算熱力學吸附常數(包括自由能、焓、熵)進行評估。各種吸附劑對Cu2+、Zn2+、及Pb2+吸附平衡模式符合Langmuir equation 及Freundlich equation。吸附動力模式符合Pseudo-second order equation。自由能(ΔG^0)皆為負數，顯示吸附為自發性反應。隨等溫吸附反應之溫度升高（15℃、30℃、50℃、及70℃），自由能(ΔG^0)之絕對值越高，顯示吸附之反應常數越大。吸附反應之焓(ΔH^0)皆為正數，顯示此吸附為吸熱反應。熵(ΔS^0)皆為正數，顯示吸附時固體吸附劑表面和金屬溶液介面間之自由度增加。

Heavy metal ions commonly found in wastewater are discharged from industries, such as metal plating facilities, mining operations and tanneries. Heavy metals, causing various diseases and disorders, are not biodegradable and tend to accumulate in aquatic organisms. Barley waste (Hordeum spp.)was chemically modified by combined treatments, either with 1% thiourea cross-linkage or without thiourea, followed by acetic acid modification to improve the physical and chemical adsorption capacity of heavy metal ions (Cu(Superscript 2+), Zn(Superscript 2+) and Pb(Superscript 2+)). Batch adsorption on chemically modified barley adsorbents have been carried out with variation in pH, contact time, temperature, and heavy metal ions concentrations. The Langmuir, Freundlich and Dubinin-Radushkevich(D-R) isotherm adsorption equilibrium models were used to assess the adsorption behavior of heavy metals. Pseudo-first-order rate, pseudo-second-order rate, and intraparticle diffusion equations used in the adsorption kinetic models were discussed. The thermodynamic parameters, including Gibbs free energy, enthalpy, and entropy of adsorption reaction were also discussed. The isotherm equilibrium adsorption of heavy metal ions fitted well with the Langmuir and Freundlich model. The experimental data showed that the pseudo-second-order rate model for Cu(Superscript 2+), Zn(Superscript 2+), and Pb(Superscript 2+) ions adsorption was excellently fitted to modified barley adsorbents and gave the best correlations (r^2＝0.98~1.0). This result indicated that chemical adsorption was the basic mechanism for this process. The results of the thermodynamic investigations indicated that the adsorption reactions were spontaneous (Gibbs free energy＜0), endothermic (enthalpy＞0), and irreversible (entropy＞0). The modified barley waste, which has a very low economical value, was proved as suitable adsorbents for the removal of heavy metals from aqueous solutions.