| Resource & environmental sciences |
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| Humic acid from leonardite for cadmium adsorption and potential applications |
| MENG Fande1,2, YUAN Guodong1*, WEI Jing1, BI Dongxue1,2, WANG Hailong3,4, LIU Xingyuan4 |
| (1. Key Laboratory of Coastal Environmental Processes and Ecological Remediation/Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences; Shandong Provincial Key Laboratory of Coastal Environmental Processes, Yantai 264003, Shandong, China; 2.University of Chinese Academy of Sciences, Beijing 100049, China; 3. School of Environmental and Resource Sciences, Zhejiang A & F University, Lin’an 311300, Zhejiang, China; 4. Guangdong Dazhong Agriculture Science Co., Ltd., Dongguan 523169, Guangdong, China) |
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Abstract Cadmium (Cd) is a toxic and carcinogenic contaminant released from a wide range of industries. Its accumulation in soil and water is of growing environmental and health concerns. Thus, there is an increasing demand for materials that are effective for Cd adsorption, economically feasible, and unlikely to create new environmental problems.
Humic acid (HA) is ubiquitous in soil and sediment. It can also be extracted from lignite and leonardite, which can be viewed as ancient biochar and oxidized ancient biochar, respectively. Being abundant in carboxyl and hydroxyl groups (—COOH, —OH), HA has a strong propensity for adsorbing heavy metal cations by forming inner-sphere complexes, thus reducing their mobility and bioavailability. A leonardite (Leo) from Xinjiang, China was used to produce HA by ultrasonically dispersing Leo in 0.1 mol/L NaOH solution at 40 ℃ for 30 min, and then flocculating the supernatant with 6 mol/L HCl. The obtained HA (Leo-HA) was characterized for its surface properties by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), and was analyzed for elemental compositions, carboxyl group, and isoelectric point (pI). Further, its adsorption characteristics were determined by kinetic and batch experiments, and its potential applications in removing Cd2+ from water and immobilizing Cd2+ in soils were assessed based on its technical reliability, economic feasibility, and environmental impact. For environmental relevance, a relatively low range of Cd2+ concentrations (0-100 mg/L) was used in adsorption experiment, during which the pH of solution was maintained constant.
Results showed that Leo-HA had a low pI of 2.7 and a high C content of 58.68%, which is not unusual for coal-derived HA. FTIR confirmed the abundance of —COOH and —OH groups. The carboxyl group was determined at 5.81 mol/kg. The low Cd concentration (0.15 mg/kg) of Leo-HA met the prerequisite for safe use in agricultural land, as regulated by Farmland Environmental Quality Standards. Kinetic studies showed that Cd2+ adsorption onto Leo-HA reached equilibrium in 8 h and the process was pH-dependent. The adsorption increased with pH within 2.0-6.0, and then decreased as pH further increased to 7.0. This is because Leo-HA starts to dissolve as solution pH approaches to neutral. Adsorption data were better fitted into Langmuir equation (R2=0.991) than Freundlich equation (R2=0.891), suggesting the monolayer nature of Cd2+ adsorption onto Leo-HA. At pH 5.0, the maximum adsorption capacity (Qm) derived from the Langmuir equation was 137.37 mg/g, which was equivalent to 71% of the carboxyl groups of Leo-HA. This Qm was much higher than what had been reported in the literature for lignite, lignite-derived HA, and soil HA. A simple comparison at pH 4.3 and an initial Cd2+ concentration of 80 mg/L showed that the Leo-HA adsorbed more Cd2+ (86.97 mg/g) than the reference HA 1R106H (73.49 mg/g) from IHSS did, even though the later had a higher carboxyl content (6.82 mol/kg). This apparent discrepancy was due to the fact that dissolution of 1R106H was observed at pH 4.3, whereas the Leo-HA was stable at this pH.
Leonardite is abundant across China. Leo-HA has the advantages of low cost, high adsorption capacity, and low Cd content, thus it is a prospective adsorbent for immobilizing Cd2+ in contaminated soils or removing Cd2+ from water. Lime has widely been used to immobilize Cd2+ in soils, but it tends to reduce soil organic matter content, damage soil structure, and pose a hazard to the safety of its users in the field. In contrast, Leo-HA is beneficial to soil structure and soil quality, as well as safe to handle in the field. Field trials of applying Leo-HA onto heavy metal contaminated soils would be a logical step to follow.
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Published: 20 July 2016
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风化煤提取的胡敏酸对镉的吸附性能及其应用潜力
以“碱溶酸析”法从新疆风化煤中提取的胡敏酸为研究对象，对其理化性质和表面形态进行表征，并通过吸附试验探究反应时间、溶液pH、镉离子（Cd2+）质量浓度对胡敏酸吸附Cd2+的影响。结果表明：风化煤提取的胡敏酸碳元素质量分数高达58.68%，羧基的质量摩尔浓度为5.81 mol/kg，等电点为2.7；该胡敏酸含Cd量为0.15 mg/kg，符合国家土壤环境质量标准。胡敏酸对Cd2+的吸附在8 h内达到平衡，吸附量随Cd2+质量浓度（0~100 mg/L）和溶液pH升高而增加，到pH=6.0时最大，之后胡敏酸开始溶解导致吸附量降低。Langmuir方程比Freundlich方程能更好地拟合胡敏酸对Cd2+的吸附等温线，显示出单分子层吸附的特点。在pH=5.0时，胡敏酸对Cd2+的饱和吸附量达137.37 mg/g，相当于用去了酸度系数(pKa)为3的羧基含量的71%。在pH=4.3、Cd2+初始质量浓度为80 mg/L的同等条件下，新疆风化煤提取的胡敏酸对Cd2+的吸附量为86.97 mg/g，高于国际腐殖质协会胡敏酸标样1R106H对Cd2+的吸附量（73.49 mg/g）。风化煤来源广、储量大、价格低，以它为原料制备获得的胡敏酸产量高、吸附能力强、环境友好、施用安全，有望作为吸附剂用于含重金属废水处理，以及作为钝化剂和土壤调理剂用于重金属污染土壤的修复。
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