Iron Dissolution Characteristics Of Eight Representative Types Of Particles In Acidic Solutions

Iron is an essential micronutrient element for all lives.In marine areas with high nutrients and low chlorophyll,the concentration of soluble iron limits the metabolism and population abundance of phytoplankton,which in turn affects marine primary productivity and the global carbon cycle.Only soluble Fe is bioavailable.However,due to the large uncertainty in aerosol iron solubility,it is difficult to quantitatively evaluate the real concentration level of soluble iron input from the atmosphere to the ocean.During atmospheric transportation,aerosols may react with acid gases（SO₂,NO_x,organic acids,etc.）,which could significantly increase iron solubility.A large number of studies have explored the impact of atmospheric aging on iron solubility.However,iron solubility is described by using various parameters,such as solubility,concentration and so on,which makes it hard for us to compare their results due to large discrepancy in experiment conditions among different research groups.In this study,we selected eight iron-bearing minerals,including hematite,goethite,illite,montmorillonite,China loess（LC）,Arizona test dust（ATD）and two types of coal fly ash（GBW08401,CFA2691）and investigated the effect of acidity（pH=2 or 3）,organic acid species（acetate,oxalate）and oxalate concentrations（0.5-4 mmol/L）on iron solubility,then a comprehensive set of parameters for iron solubility was obtained.In addition,this study also compared the differences of iron solubility among different particulates,and illustrated the differences with specific surface area,particle size,iron speciation and mineral composition.The main conclusions are as follows:（1）pH significantly affects iron solubility of hematite.When the pH was increased by one unit,the iron solubility of hematite decreased from 0.43±0.06%（pH=2）to less than 0.01%（pH=3）after 48 h of dissolution,and the proton-promoted dissolution efficiency decreased significantly.Comparing with the effect of different organic acids on the solubility of iron,it was found that the addition of acetate didn’t promote the dissolution of iron,but oxalate did.In addition,the iron solubility of hematite had a positive correlation with oxalate concentrations（0-4 mmol/L）.When pH was 3,the iron solubility of hematite decreased by 55.3%in the solution containing 2 mmol/L oxalate,compared with that its pH was 2,implying that the increase in pH value directly inhibits proton-promoted dissolution and indirectly inhibits ligand-promoted dissolution.（2）After reacting in sulfuric acid solution（pH=2）for 48 h,the iron solubilities were in the following order:CFA2691（43.72±6.56%）>LC（12.58±0.76%）>ATD（5.03±0.43%）>GBW08401（3.03±0.27%）>illite（2.18±0.14%）>goethite（1.68±0.18%）>hematite（0.43±0.06%）>montmorillonite（0.20±0.08%）.In general,iron solubility of these iron-containing minerals was as follows:coal fly ash>mineral dust>clay minerals≈iron（hydro）oxides.With the increase of oxalate concentrations（0.5-2 mmol/L）,the corresponding increase in iron solubility was in the following order:iron（hydro）oxides>clay minerals,mineral dust>coal fly ash.It was probably because most of the soluble iron in clay minerals and coal fly ash had already dissolved via proton-promoted dissolution,so the effect of ligand-promoted dissolution seemed minor.On the contrary,for the iron（hydro）oxides and mineral dust,the effect of ligand-promoting dissolution has a positive relationship with the concentration of oxalate.（3）We further investigated the dependence of the iron solubility of eight typical iron-bearing particles（pH=2,H₂SO₄,48 h）on their corresponding particle size and specific surface area.It seemed that iron solubility had no correlation with particle size and specific surface area.In addition,the mineral composition couldn’t explain the differences.However,this study found that the iron solubility was significantly correlated with iron speciation,which meaned the more amorphous iron account for total iron,the higher iron solubility was.Combined with the extraction results of carbonate iron,this study found that the content of carbonate iron was in the order:CFA2691（54.2±3.8%）>LC（10.3±0.2%）>ATD（4.9±0.0%）>GBW08401（2.9±0.2%）,which was in good agreement with the iron solubility,indicating that carbonate iron was an important source of soluble iron under strong acidic conditions.