Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO, USA.
Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO, USA.
Water Res. 2020 May 15;175:115688. doi: 10.1016/j.watres.2020.115688. Epub 2020 Mar 5.
Ion exchange is widely used to treat nitrate-contaminated groundwater, but high salt usage for resin regeneration and management of waste brine residuals increase treatment costs and add environmental burdens. Development of palladium-based catalytic nitrate treatment systems for brine treatment and reuse has showed promising activity for nitrate reduction and selectivity towards the N over the alternative product ammonia, but this strategy overlooks the potential value of nitrogen resources. Here, we evaluated a hybrid catalytic hydrogenation/membrane distillation process for nitrogen resource recovery during treatment and reuse of nitrate-contaminated waste ion exchange brines. In the first step of the hybrid process, a Ru/C catalyst with high selectivity towards ammonia was found to be effective for nitrate hydrogenation under conditions representative of waste brines, including expected salt buildup that would occur with repeated brine reuse cycles. The apparent rate constants normalized to metal mass (0.30 ± 0.03 mM min g under baseline condition) were comparable to the state-of-the-art bimetallic Pd catalyst. In the second stage of the hybrid process, membrane distillation was applied to recover the ammonia product from the brine matrix, capturing nitrogen as ammonium sulfate, a commercial fertilizer product. Solution pH significantly influenced the rate of ammonia mass transfer through the gas-permeable membrane by controlling the fraction of free ammonia species (NH) present in the solution. The rate of ammonia recovery was not affected by increasing salt levels in the brine, indicating the feasibility of membrane distillation for recovering ammonia over repeated reuse cycles. Finally, high rates of nitrate hydrogenation (apparent rate constant 1.80 ± 0.04 mM min g) and ammonia recovery (overall mass transfer coefficient 0.20 m h) with the hybrid treatment process were demonstrated when treating a real waste ion exchange brine obtained from a drinking water utility. These findings introduce an innovative strategy for recycling waste ion exchange brine while simultaneously recovering potentially valuable nitrogen resources when treating contaminated groundwater.
离子交换广泛用于处理硝酸盐污染的地下水，但树脂再生和废盐水残渣管理需要大量的盐，这增加了处理成本并增加了环境负担。开发基于钯的催化硝酸盐处理系统用于盐水处理和再利用，已经显示出对硝酸盐还原和对 N 的选择性具有很大的活性，超过了替代产物氨，但这种策略忽略了氮资源的潜在价值。在这里，我们评估了一种混合催化加氢/膜蒸馏工艺，用于处理和再利用硝酸盐污染的废离子交换盐水时回收氮资源。在混合工艺的第一步中，发现 Ru/C 催化剂对氨具有高选择性，可在代表废盐水的条件下有效地进行硝酸盐加氢，包括预期会在重复盐水再利用循环中发生的盐积累。根据金属质量归一化的表观速率常数（在基线条件下为 0.30±0.03 mM min g）与最先进的双金属 Pd 催化剂相当。在混合工艺的第二步中，应用膜蒸馏从盐水基质中回收氨产物，将氮捕获为硫酸铵，这是一种商业肥料产品。溶液 pH 通过控制溶液中游离氨（NH）的分数显著影响氨气通过透气膜的传质速率。盐水盐度的增加不会影响氨的回收率，表明膜蒸馏在重复再利用循环中回收氨的可行性。最后，当处理来自饮用水处理厂的实际废离子交换盐水时，该混合处理工艺表现出高的硝酸盐加氢速率（表观速率常数为 1.80±0.04 mM min g）和氨回收率（总传质系数为 0.20 m h）。这些发现为回收废离子交换盐水的同时，当处理受污染的地下水时，提供了一种同时回收有价值氮资源的创新策略。
