Impacts Of Wildfire Ash On The Fate Of Mercury And The Mechanisms In Aquatic Environment

Mercury（Hg）is a ubiquitous contaminant in the environment and its methylated form,methylmercury（Me Hg）,leads to extensive bioaccumulation and biomagnification in aquatic food webs,and causes the threat of the quality and safety of agricultural products and human health.Meanwhile,wildfires are expected to become more frequent and intensive at the global scale due to climate change,which not only destroy the balance of forest ecosystems,but also adversely affect industrial and agricultural production.As a new material present after a wildland fire,wildfire ash（WA）can have profound effects on ecosystems.It affects biogeochemical cycles and cause long-term impacts on agricultural environment,but in the present their interactions with aqueous and sedimentary Hg are poorly understood.Herein,A batch adsorption experiment was conducted to compare the differences of wildfire ash（Wragg wildfire,WA1 and Rocky wildfire,WA2）with activated carbon（AC）,biochar（high temperature,BC1 and low temperature,BC2）and lab controlled-burn ash（pyrolysis 250℃:CA1,pyrolysis 550℃:CA2,thermal oxidation250℃:CA3 and thermal oxidation 550℃:CA4）on the sorption of aqueous inorganic Hg and explore the underlying mechanisms and the effects of combustion conditions and environmental factor of Hg sorption by wildfire ash.Meanwhile,sediment incubation experiments were used to assess how wildfire ash affected the sedimentary Hg methylation and bioavailability.Finally,QWASI（Quantitative Water-Air-Sediment Interaction）model was selected to estimate the fate and transport of mercury in the downstream watersheds before and after wildfire ash input,in order to provide theoretical and scientific basis for the post-fire aquatic ecosystem restoration.The main results were summarized as follows:（1）Wildfire ash sequestered aqueous Hg efficiently as AC and BC1.Specifically,the similar adsorption efficiencies（>99.0%）of Hg（Ⅱ）were observed for WA1,WA2,AC and BC1 as well as the kinetics data were better fitted with the pseudo-second-order kinetic model than other models with the similar fitted equilibrium value(0.20μg g^-1).Meanwhile,the maximal Hg（Ⅱ）adsorption capacity of WA1(1.92μg g^-1)and WA2(1.87μg g^-1)was no significant difference with AC(1.87μg g^-1)and BC1(1.95μg g^-1)（p>0.05）.And the strong Freundlich model fit suggested that the adsorption process for Hg（Ⅱ）by wildfire ash proceeds as chemisorption on heterogeneous surface.（2）The adsorption of aqueous Hg（Ⅱ）by wildfire ash samples were mainly attributed to the formation of complexes between Hg（Ⅱ）and oxygen-containing functional groups,and the co-precipitation with mineral components.Specifically,the results of the normalization coefficient of organic carbon(log K_OC)showed that log K_OC of WA1,WA2and BC1 was significant negative correlation with the ratio of H/C and（O+N）/C in low mercury freshwater（p<0.05）,while exhibited significant positive correlation with the ratio of H/C and（O+N）/C in high mercury freshwater(～10μg L^-1)（p<0.05）,indicated that the sorption of Hg（Ⅱ）by wildfire ash and BC1 was mainly attributed to the complexation of Hg（Ⅱ）and the carbon functional groups in the surface in low mercury environment,while there could be other mechanisms of wildfire ash with mercury increasing such as co-precipitation with mineral components as the higher ash contents in wildfire ash than BC1.Furthermore,X-ray photoelectron spectroscopy displayed that a similar Hg（Ⅱ）adsorption mechanism for wildfire ash,activated carbon and biochar was found that involves the formation of complexes between Hg（Ⅱ）and carboxylic group.Unlike activated carbon and biochar,the oxygen content in the C-O/-OH group decreased after Hg（Ⅱ）adsorption by wildfire ash,indicated that phenolic hydroxyl groups might participate in the Hg（Ⅱ）adsorption by wildfire ash.（3）Wildfire ash sequestered aqueous Hg（Ⅱ）but caused an initial increase in p H,metal and DOC content in natural freshwater.Specifically,compared with the background values,the p H and DOC values of natural freshwater increased by 8.85%and 4.20%,0.58times and 0.74 times with the addition of WA1 and WA2,respectively（p<0.05）.Meanwhile,the metal contents such as K,Ca,Mg,and Fe increased by 3.51-24.6 mg L^-1with the addition of wildfire ash.However,the adsorption efficiency of Hg（Ⅱ）in natural freshwater of WA1（44.9%）and WA2（43.7%）was significantly lower than AC（79.5%）and BC1（75.8%）（p<0.05）.The pseudo-second-order kinetic model was more suitable for the adsorption behavior of WA1 and WA2 with the fitted equilibrium value of 0.18μg g^-1and 0.16μg g^-1.Meanwhile,the maximal adsorption capacity of Hg（Ⅱ）by WA1(1.56μg g^-1)and WA2(1.41μg g^-1)was significantly lower than AC and BC1 in natural freshwater（p<0.05）.This results indicated that the complex components of natural freshwater cause the adverse effects of Hg（Ⅱ）adsorption by wildfire ash.（4）Dissolved organic carbon（DOC）increasing remarkably reduced the Hg（Ⅱ）adsorption by wildfire ash,while the adsorption capacities of aqueous Hg（Ⅱ）by lab-controlled burning ash samples were enhanced with the increase of temperature and oxygen availability.Specifically,compared with CA1,adsorption efficiency of CA2 and CA3 increased by 61.0%and 64.4%though the increase of temperature and oxygen availability,respectively.Moreover,the adsorption efficiency of Hg（Ⅱ）by WA1,WA2,AC,and BC1 reduced by 30.7%,46.6%,17.0%,and 17.5%with DOC addition,respectively.Notably,DOC had strong effects on the adsorption capacities of wildfire ash samples when compared to that of AC and BC1.Moreover,the conditional stability constants（log K）between DOC and aqueous Hg（Ⅱ）showed that the 1:2 Hg-DOC complexes inhibited Hg（Ⅱ）adsorption by wildfire ash with（-COO）₂Hg complexes,which were attributed to the presence of labile DOC from wildfire ash in the adsorption process.（5）Wildfire ash could strongly sequester dissolved mercury（DHg）,but might mobilize sedimentary Hg and/or promoted the production of methylmercury（Me Hg）.Specifically,compared with CK,WA1 and WA2 significantly reduced the content of DHg with the values 38.3%and 33.8%,respectively,while Me Hg in water significantly increased by2.78 times and 2.18 times in low Hg sediment sealed incubation,respectively（p<0.05）.Meanwhile,compared with CK,the logarithmic sediment-water partition coefficients of inorganic Hg(log K_{d IHg})increased by 5.13%and 3.64%,respectively,while log K_{d Me Hg}reduced by 10.1%and 9.54%,respectively,implied that more IHg deposited from water to sediment while more Me Hg released from sediment to water with the addition of WA1 and WA2.Additionally,compared with CK,DHg significantly reduced by 86.4%and 88.2%while Me Hg and DOC in water significantly increased by 2.46 times,2.10 times and 1.19times and 1.43 times with 5%addition of WA1 and WA2 in high Hg sediment sealed incubation,respectively（p<0.05）.Meanwhile,compared with CK,the log K_{d IHg}increased by 25.2%and 26.8%with the addition of WA1 and WA2,but there were not significant differences of log K_{d Me Hg}.The results may imply that wildfire ash may disproportionately increase the availability of inorganic Hg for microbial Hg methylators and may result in higher inorganic Hg methylation yields under elevated DOC conditions.Interestingly,compared with CK,relatively high levels of total mercury（THg）were reduced from earthworm（Lumbricus terrestris）with the addition of WA1,WA2,AC and BC1 with a range of 16.6%-43.7%,24.1%-41.7%,61.1-94.1%,and 25.5-63.1%after four weeks opens incubation,respectively.Meanwhile,BAF values of WA1,WA2,AC and BC1 treatments were all below one,implying that THg was not generally bioaccumulate in earthworms with the addition of these materials.However,compared with CK,Me Hg levels of earthworms were increased by 4.37%-114%and 6.57-102%with the addition of WA1 and WA2,respectively.Although the increase amplitude was smaller with time,higher BAF（>1）indicated that wildfire ash might promote the bioaccumulation of Me Hg in earthworms.（6）Wildfire ash increased the migration flux of mercury of suspended particles deposition and outflow process,and promoted more Hg²⁺and Me Hg transfer from water to sediment.Specifically,the results of QWASI model showed that the net balance of Hg⁰,Hg²⁺,and Me Hg in the target watershed was increasing after wildfire.Notably,with wildfire ash input the net transmission direction of Hg⁰ was from sediment to water with net transmission rate of 8.67×10⁵ g,and the net transmission direction of Hg²⁺and Me Hg was from water to sediment with net transmission rate of 1.39×10⁷ g and 2.26×10⁶ g,respectively.Overall,the model output demonstrated that the contents of mercury in the downstream watersheds were increased after wildfire.Therein,the input of wildfire ash changed the environmental fate of mercury.However,to ensure the accuracy of the model output,the higher impact parameters were obtained through measurement in the future study.