Fluoride adsorption, transport and modelling in volcanic soils of Iceland for future risk assessments

大约225年前，1万冰岛人——大约五分之一——死亡，岛上的大部分牲畜都死了。这场灾难是人类开始占领冰岛以来对冰岛影响最大的一次，它是由一个被称为拉基加尔的火山裂缝系统的觉醒造成的。在几个月的时间里，Lakakigar向空气中喷出了大量的有毒酸性气体和颗粒，氟化物是这些排放物中明显的一部分。氟化物因其对植物、动物和人类的有害影响而闻名；几个世纪以来，饮用水中过量的汞对人类和牲畜的健康构成了威胁。与氟化物过度接触有关的影响包括儿童牙齿矿化障碍和氟骨症，这是一种以骨骼异常生长为特征的潜在致命疾病。最近，有一种观点认为，在Lakakigar火山喷发期间，地表和地下水中持续存在高浓度的火山源氟化物，最终导致人类中毒。如果这是正确的，冰岛的裂缝喷发可能比科学家们想象的要危险得多。这就是为什么研究人员认为迫切需要为冰岛下一次大型火山事件做准备，这将威胁生命并造成深远的社会经济影响。氟化物污染理论的一个大问题是，通过雨水和火山灰沉积在地面上的火山源氟化物排放必须先通过土壤剖面滤出，然后才能到达地下水。然而，氟化物是一种非常活跃的离子，很容易与某些矿物成分相互作用，并积聚在某些矿物成分的表面上，这些矿物成分通常存在于火山物质（或火山土壤）中，例如在冰岛发现的那些。一般来说，这种所谓的化学吸附过程减弱了污染物在土壤剖面中的迁移，从而减少了对地下水构成的危险。然而，一些复杂的因素可能改变吸附并降低其强度。例如，土壤环境中的酸度水平强烈影响氟化物的吸附。此外，一种化学物质，如硫酸盐，通常在火山沉积中与氟化物共存，可能会“隐藏”氟化物倾向粘附的矿物表面。最后，氟化物可能只是没有足够的时间与土壤中的矿物质结合，因为运输氟化物的液态水在土壤中流动得太快了。氟化物在土壤上的吸附通常使用经验模型来描述，即通过调整一个或多个参数来拟合实验数据的方程。这些模型是特定于所研究的系统的，因此，它们不能用于预测不同土壤和环境条件下氟化物的命运。要解决冰岛火山土壤中富氟化物火山沉积的迁移问题，需要一个具有坚实理论基础的新的建模框架。这个项目的主要目的是开发和测试一个基于表面化学的研究模型，同时进行基于实验室的实验工作。该模型将使用“假设”问题来探讨冰岛代表性土壤中氟化物运输的各种情况。如果模拟数据显示氟化物可以被浸出，那么地下水污染的风险将得到明确的证明。这一发现将对冰岛和其他地方的大型裂缝型喷发具有重要的环境意义。相反，如果结果表明土壤中氟化物含量很高，那么通过饮水产生的氟化物危害可能会减少，但土壤的化学性质可能会发生很大变化。在未来，该模型可以进一步发展，以研究其他潜在有害的无机物质（如硫酸盐和金属）在火山土壤中的迁移。

Around 225 years ago, 10,000 Icelanders - roughly one in five - died and most of the livestock on the island perished. This catastrophe, the greatest to affect Iceland since human occupation began, was caused by the awakening of a volcanic fissure system, known as Lakakigar. During several months, Lakakigar erupted prodigious quantities of noxious acidic gases and particles into the air, and fluoride was a conspicuous part of these emissions. Fluoride is renowned for its harmful effects on plants, animals and humans; its occurrence in excessive amounts in drinking water has constituted a health threat for people and livestock for centuries. Effects linked to fluoride overexposure include mineralization disorder of the teeth in children and skeletal fluorosis, a potentially fatal disease characterized by abnormal bone growths. Recently, the idea has emerged that during the Lakakigar eruption high levels of volcanogenic fluoride persisted in surface and groundwater, ultimately leading to poisoning in people. If this is correct, Iceland's fissure eruptions may be much more dangerous than scientists had envisaged. This is why researchers see an urgent necessity to prepare for the next large volcanic event in Iceland, which will threaten lives and result in profound socio-economic impacts. One of the big problems with the fluoride pollution theory is that volcanogenic fluoride emissions that are deposited onto the ground via rain water and volcanic ash must leach through the soil profile before reaching groundwater. However, fluoride is a very reactive ion that readily interacts with, and accumulates onto the surface of, certain mineral constituents commonly present in soils derived from volcanic material (or volcanic soils), such as those found in Iceland. In general, this so-called chemical adsorption process attenuates migration of a contaminant in the soil profile, thereby reducing the risk posed to groundwater. However, several complicating factors may alter adsorption and diminish its intensity. For example, the acidity level in the soil environment strongly influences adsorption of fluoride. Also, a chemical species such as sulphate, which typically coexists with fluoride in volcanogenic deposition, may 'hide' the mineral surfaces onto which fluoride tends to stick. Finally, fluoride may just not have enough time to bind to the soil's minerals, because the liquid water which transports it flows too rapidly through the soil. Adsorption of fluoride on soils has been classically described using empirical models, whereby an equation is fitted to experimental data by adjusting one or more parameters. These models are specific to the system under study and as such, they cannot be used to predict the fate of fluoride under diverse soil and environmental conditions. What is needed to address the question of the transport of fluoride-rich volcanogenic depositions in the volcanic soils of Iceland is a new modelling framework with a robust underpinning theoretical basis. The primary aim of this project is to develop and test a surface chemistry-rooted research model in parallel with laboratory-based experimental work. The model will be interrogated using 'what if' questions to explore various scenarios of fluoride transport through representative soils of Iceland. If the simulation data reveal that fluoride can be leached, then the risk of groundwater pollution would be demonstrated unequivocally. This finding would have important environmental implications for large fissure-type eruptions in Iceland and elsewhere. Conversely, if the results indicate that fluoride is strongly held in soils, then the fluoride hazard through ingestion of water may be reduced but the chemical properties of the soils may be greatly altered. In the future, the model could be developed further to study the transport of other potentially harmful inorganic substances (e.g., sulphate and metals) in volcanic soils.