Chemical Engineering Journal: 水热预处理联合改性水热炭吸附技术回收沼液...
中国农业大学资源与环境学院张涛副教授等采用水热预处理联合改性水热炭吸附技术回收沼液中氨氮。其中水热预处理可实现不溶性或有机氮向无机氮的转化，而碱活化水热炭显著提高了氨氮的吸附效率。通过建立吸附动力学与热力学模型，定量评价吸附手段下氨氮的回收效能。相关成果发表于Chemical Engineering Journal (IF=10.652)。
该研究由中国农业大学资源与环境学院联合德国伍珀塔尔大学，南京大学共同完成。近年，张涛副教授在农业废弃物养分循环与利用领域开展了深入的研究，主要研究成果在Green Chemistry, Chemical Engineering Journal, Bioresource Technology, Journal of Cleaner Production, Science of the Total Environment, Chemosphere, Waste Management 等学术期刊发表。研究获得国家重点研发计划、国家自然科学基金等项目的资助。
联系作者：张涛，副教授，博士生导师，中国农业大学，Email: taozhang@cau.edu.cn
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·Up to 93.45% of total nitrogen transformed into ammonium by hydrothermal process.
·Activated hydrochar was synthesized to capture ammonium nitrogen.
·10.14% of total nitrogen was recovered after solubilization, capture, and available.
·Nitrogen sorption by activated hydrochar was assessed by FTIR, SEM and BET.
Nitrogen recovery from bio-based digestate has attracted considerable interests recently, but has been limited by the indirect capture of organic or insoluble nitrogen and the lower sorption ability of hydrochar. To mitigate these challenges, we used a hydrothermal pretreatment to promote the conversion of insoluble or organic nitrogen. Activated hydrochar was synthesized by using potassium hydroxide to enhance its sorption capacity, and the hydrochar was characterized by Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller surface-area analysis, and scanning electron microscopy. An optimal efficiency was achieved at 210 °C, with 93.45% total nitrogen solubilization into ammonium. The sorption data of ammonium on the activated hydrochar fitted the pseudo second order kinetics and Langmuir model well. This reaction was dominated by chemisorption, which referring to the formation of bonds between (oxygen-containing) functional groups and ammonium. The maximum ammonium adsorption on the activated hydrochar at 260 °C (260AH) was 108.57 mg g−1. The ammonium sorption was endothermic, spontaneous and associated with a disorder increase on the solid-liquid interface. Once saturated, the ammonium was released continually from activated hydrochar and is anticipated to show an optimal performance in an alkaline environment. The amount of ammonium that was released from the saturated activated hydrochar at 260 °C after five extractions was 13.09 mg g−1. The nitrogen mass-balance calculations show that 10.14% total nitrogen was recovered after nitrogen solubilization, capture, and release. Therefore, potassium-hydroxide-activated hydrochar performed as a promising composite sorbent for ammonium.
氮的应用对我国农业发展起到了巨大的推动作用，但同时也是一把双刃剑。以农业生物基质发酵产物——沼液为例，随着我国大中型沼气工程的建设及投入使用，大量富含氮素的沼液产生，如不加以无害化处理和资源化肥料利用，既会对水环境产生严重污染，又会造成养分资源的巨大浪费，将成为我国现代农业绿色循环发展的制约性因素。如何对农业废弃物中盈余氮素进行回收成为研究热点，其中克服废液中氮素形态（有机或不溶性氮）这一关键制约因素也是研究的难点。
本研究通过水热预处理沼液实现含氮化合物的转化及释放，定性解析了水热炭改性的化学反应过程，从动力学及热力学模型拟合层面揭示了溶液中含氮化合物的吸附机制，最后对饱和吸附样品中氮的有效性进行了评估。
水热预处理沼液释放氮素实验以pH=3的沼液调试液为底物，设置120, 150, 180 ℃和 210 ℃温度梯度组，即HD120, HD150, HD180和HD210。由3D-EEM分析结果可知水热预处理通过水解蛋白质、降解有机质，细胞裂解等方式促进了粪便中氮的溶解和释放。本组实验在210 ℃水热处理中实现了溶解和转化铵态氮的最佳性能，氨氮转化率达到93.45％。
3D-EEM分析不同水热温度处理下的沼液产物 (a) RD, (b) HD120, (c) HD150, (d) HD180, (e) HD210.
在吸附剂制备的过程中，较高温下生成的水热炭（260AH）表面破碎程度大、粗糙，表面裂纹较多。这是因为纤维素结构在较高的温度下更容易受到破坏，从而使原始孔隙降解形成裂纹。且在KOH活化过程中，一些堵塞孔隙的杂质被分解或挥发，从而将杂质从裂缝中去除。这对吸附性能的提高具有一定的促进作用。
SEM分析(a) 230RH, (b) 230AH, (c) 260RH, (d) 260AH, (e) 260CM水热炭样品的表面形态结构
吸附实验表明，活性水热炭对氨盐的吸附数据符合拟二阶动力学（pseudo second order kinetics）和朗缪尔模型（Langmuir model）。即反应以化学吸附为主，吸附量最高可达108.57 mg·g−1。吸附机理如下图所示。
吸附机理示意图
在连续浸提实验过程中，饱和吸附的260PAH样品释放效率为pH8.5 > 6.5 > 4.5(图a)。采用拟二阶动力学法对数据进行拟合，发现在碱性条件下该样品得到了最佳的释放效率（120 h时达到9.24 mg·g−1）。在间隔浸提过程中，饱和吸附的260PAH样品在pH8.5条件下氮释放量达到13.09 mg·g-1，这表明饱和吸附水热炭具有可用作氨基肥料的潜力，在农业领域应用过程中达到可持续性的目标。
260PAH饱和吸附氮的有效性分析：a连续浸提；b间隔浸提；c间隔浸提下氮的累积利用率
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校对和审核：张阳 王农