DiMento Brian P, Mason Robert P
University of Connecticut, Department of Marine Sciences, 1080 Shennecossett Rd, Groton, CT 06340, United States.
Mar Chem. 2017 Nov 20;196:116-125. doi: 10.1016/j.marchem.2017.08.006. Epub 2017 Aug 14.
Many studies have recognized abiotic photochemical degradation as an important sink of methylmercury (CHHg) in sunlit surface waters, but the rate-controlling factors remain poorly understood. The overall objective of this study was to improve our understanding of the relative importance of photochemical reactions in the degradation of CHHg in surface waters across a variety of marine ecosystems by extending the range of water types studied. Experiments were conducted using surface water collected from coastal sites in Delaware, New Jersey, Connecticut, and Maine, as well as offshore sites on the New England continental shelf break, the equatorial Pacific, and the Arctic Ocean. Filtered water amended with additional CHHg at environmentally relevant concentrations was allowed to equilibrate with natural ligands before being exposed to natural sunlight. Water quality parameters - salinity, dissolved organic carbon, and nitrate - were measured, and specific UV absorbance was calculated as a proxy for dissolved aromatic carbon content. Degradation rate constants (0.87-1.67 day) varied by a factor of two across all water types tested despite varying characteristics, and did not correlate with initial CHHg concentrations or other environmental parameters. The rate constants in terms of cumulative photon flux values were comparable to, but at the high end of, the range of values reported in other studies. Further experiments investigating the controlling parameters of the reaction observed little effect of nitrate and chloride, and potential for bromide involvement. The HydroLight radiative transfer model was used to compute solar irradiance with depth in three representative water bodies - coastal wetland, estuary, and open ocean - allowing for the determination of water column integrated rates. Methylmercury loss per year due to photodegradation was also modeled across a range of latitudes from the Arctic to the Equator in the three model water types, resulting in an estimated global demethylation rate of 25.3 Mmol yr. The loss of CHHg was greatest in the open ocean due to increased penetration of all wavelengths, especially the UV portion of the spectrum which has a greater ability to degrade CHHg. Overall, this study provides additional insights and information to better constrain the importance of photochemical degradation in the cycling of CHHg in marine surface waters and its transport from coastal waters to the open ocean.
许多研究已认识到非生物光化学降解是阳光照射的地表水相中甲基汞（CHHg）的一个重要汇，但对其速率控制因素仍知之甚少。本研究的总体目标是通过扩大所研究水体类型的范围，增进我们对光化学反应在各种海洋生态系统地表水相中CHHg降解过程中相对重要性的理解。实验使用了从特拉华州、新泽西州、康涅狄格州和缅因州沿海站点以及新英格兰大陆架断裂处、赤道太平洋和北冰洋的近海站点采集的地表水。经环境相关浓度的额外CHHg修正后的过滤水，在暴露于自然阳光之前，先与天然配体达到平衡。测量了水质参数——盐度、溶解有机碳和硝酸盐，并计算了特定紫外吸光度作为溶解芳香碳含量的替代指标。尽管各测试水体特征各异，但降解速率常数（0.87 - 1.67天⁻¹）在所有测试水体类型中相差两倍，且与初始CHHg浓度或其他环境参数无关。以累积光子通量值表示的速率常数与其他研究报道的值范围相当，但处于较高水平。进一步研究反应控制参数的实验发现，硝酸盐和氯化物影响不大，且溴化物可能参与反应。使用HydroLight辐射传输模型计算了三个代表性水体——沿海湿地、河口和开阔海洋——中随深度变化的太阳辐照度，从而确定水柱积分速率。还对三种模型水体类型从北极到赤道的一系列纬度上因光降解导致的甲基汞年损失量进行了建模，得出全球去甲基化速率估计为25.3 mmol yr⁻¹。由于所有波长尤其是具有更强CHHg降解能力的紫外光谱部分的穿透增加，开阔海洋中CHHg的损失最大。总体而言，本研究提供了更多见解和信息，以更好地界定光化学降解在海洋地表水相中CHHg循环及其从沿海水体向开阔海洋输送过程中的重要性。
