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

大约225年前，10000名冰岛人--大约五分之一--死亡，岛上的大多数牲畜都死亡了。这场灾难是自人类占领以来影响冰岛的最大灾难，是由被称为Lakakigar的火山裂隙系统的觉醒引起的。在几个月的时间里，Lakakigar向空气中喷发了大量有毒的酸性气体和颗粒，而氟化物是这些排放的一个明显部分。氟化物以其对动植物和人类的有害影响而闻名；几个世纪以来，饮用水中过量的氟化物对人和牲畜的健康构成了威胁。与过量接触氟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，出现了一种想法，即在拉卡基加火山喷发期间，地表和地下水中持续存在高水平的火山成因氟化物，最终导致人中毒。如果这是正确的，冰岛的裂缝喷发可能比科学家想象的要危险得多。这就是为什么研究人员认为迫切需要为冰岛下一次大规模火山事件做准备，这将威胁生命并导致深远的社会经济影响。氟化物污染理论的一大问题是，通过雨水和火山灰沉积到地面上的火山成因氟化物排放在到达地下水之前必须通过土壤剖面淋洗。然而，氟是一种非常活跃的离子，很容易与某些矿物成分相互作用，并积累在土壤表面，这些矿物成分通常存在于来自火山物质(或火山土壤)的土壤中，例如在冰岛发现的那些土壤。一般来说，这种所谓的化学吸附过程会减弱污染物在土壤剖面中的迁移，从而降低对地下水构成的风险。然而，几个复杂的因素可能会改变吸附并降低其强度。例如，土壤环境中的酸度对氟的吸附有很大的影响。此外，在火山成因沉积中通常与氟化物共存的一种化学物种，如硫酸盐，可能会“隐藏”氟化物倾向于粘在的矿物表面。最后，氟化物可能没有足够的时间与土壤中的矿物质结合，因为输送氟化物的液态水在土壤中流动得太快。土壤对氟化物的吸附是用经验模型来描述的，即通过调整一个或多个参数来将方程与实验数据进行拟合。这些模型特定于所研究的系统，因此，它们不能用来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火山沉积的运输问题，需要有一个有坚实理论基础的新的模拟框架。这个项目的主要目的是开发和测试一个以表面化学为基础的研究模型，同时进行基于实验室的实验工作。该模型将通过“假设”问题来探讨氟化物在冰岛典型土壤中迁移的各种情景。如果模拟数据显示氟化物可以被淋溶，那么地下水污染的风险就会得到明确的证明。这一发现将对冰岛和其他地方的大型裂缝型喷发产生重要的环境影响。相反，如果结果表明氟化物在土壤中被强烈持有，那么通过摄入水而造成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的变化。今后，可以进一步开发该模型来研究其他潜在有害无机物(如硫酸盐和金属)在火山土中的迁移。

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.