Study On The Dissolution Kinetics Of Olivine In Seawater And Its Potential Carbon Sink Enhancement In Offshore Waters

Since the industrial revolution,the concentration of CO2 in the atmosphere has continued to rise,leading to major environmental problems such as global warming and ocean acidification."Carbon neutrality" is a goal of all mankind to address climate change,which can be achieved in two ways,namely reducing CO2 emissions(emission reduction)and increasing CO2 storage(increasing sinks).In order to limit global warming to the target range of 1.5～2℃set by the 2015 United Nations climate agreement,it is not enough to reduce greenhouse gas emissions,but also to actively capture and sequester CO2 from the atmosphere,so the international scientific community has proposed the implementation of atmospheric CO2 removal technology.The ocean covers 71%of the global surface area and is the largest active carbon reservoir on Earth.Studies have shown that weathering of alkaline minerals such as silicates is one of the most promising solutions.Olivine is often used as the preferred mineral for experimental research due to its wide abundance,fast dissolution rate and large CO2 storage potential.At present,there are many model studies on the dissolution rate and carbon sequestration of olivine,but there is a relatively lack of experimental research on the dissolution rate of olivine in natural seawater,and the research on ocean alkalinity enhancement in China’s offshore waters is basically blank.In this study,the dissolution kinetics of olivine(median particle size is 29 μm)in natural seawater under different conditions in the laboratory was carried out to explore the dissolution kinetics of olivine in natural seawater,and a numerical model of olivine dissolution rate constant under different conditions was constructed.Through the ship-based olivine addition experiment in the Northern Yellow Sea,the influence of olivine addition on the seawater carbonate system and its potential for carbon sequestration were explored.The main conclusions are as follows:(1)The experimental results of olivine dissolution kinetics in natural seawater in the laboratory showed that within the range of conditions set by this study(temperature T:15～35 ℃,seawater salinity S:25～34.5,seawater pH:7.7～8.1,seawater initial pCO2:220～970 ppm,stirring speed n:0～100 rpm),the dissolution rate of olivine in seawater was positively correlated with temperature,seawater pCO2 and stirring speed,that is,the higher the temperature,seawater pCO2,and stirring speed,the greater the dissolution rate of olivine;It is negatively correlated with seawater salinity and seawater pH,that is,the lower the seawater salinity and seawater pH,the greater the dissolution rate of olivine.Seawater pCO2 indirectly affects the dissolution rate of olivine by changing the pH value of seawater,and seawater pH value is the main control factor affecting the dissolution of olivine in seawater.(2)During the experiment under controlled conditions in the laboratory,olivine was dissolved in seawater non-stoichiometrically,and no secondary phase precipitation such as calcium carbonate was generated during the experiment.The dissolution rate constant of olivine was calculated by the growth rate of alkalinity of the solution,and the numerical model of olivine dissolution rate constant was constructed by coupling temperature(T),salinity(S),seawater pH and stirring speed(n):(3)The experimental results of olivine addition showed that the dissolution of olivine increased seawater total alkalinity and dissolved inorganic carbon and decreased pCO2,resulting in the ocean absorbing more CO2 from the atmosphere through sea-air exchange.On the other hand,it increases the pH of seawater to alleviate ocean acidification to a certain extent.The addition of 2 ‰ olivine had a more significant effect on the parameters of seawater carbonate system than 1‰ olivine,but the dissolution rate of olivine with large addition may slow down in the later stage of the experiment.(4)The olivine dissolution rate constant calculated based on alkalinity in the ship-based olivine addition experiment was 0.72±0.04 μmol m-2 d-1,assuming that olivine was completely removed from the water body after settlement due to gravity,and the amount of CO2 absorbed by 1 ton of olivine per unit area of seawater in the Northern Yellow Sea was only 9.8×10-5 tons.Therefore,if the scheme of adding olivine to enhance the carbon sink in offshore waters is implemented,it is necessary to consider reducing the particle size of olivine,to prolong the sedimentation time of olivine in seawater;and spreading olivine in the strongly mixed sea,to extend the contact time between olivine and seawater through repeated resuspension,so as to improve its carbon sequestration efficiency.