Dissolution of iron oxides including goethite (a-FeOOH) in aqueous solutions plays a key role in a variety of phenomena from materials corrosion to biogeochemical cycling of elements in the environment. Previous experimental studies have made great strides towards better macroscopic understanding of iron-oxide nanoparticle growth and dissolution; however, atomistic picture of these processes is still elusive. Here, we carry out detailed ab initio molecular dynamics (AIMD) based simulations of iron dissolution from the (021) and (110) goethite surfaces. We unveil the mechanistic pathways and rates of both reductive and nonreductive dissolution of iron from both surfaces in aqueous solution at room temperature. Our results clearly demonstrate that the Fe(III) reduction to Fe(II) yields much higher dissolution rates than the nonreductive pathway, while the most rapid dissolution is expected for the proton-assisted reductive mechanism. We also discuss the role of both internal (structural) and external (from solution) protonation as the way to disrupt the breaking Fe-O bonds, as well as to stabilize intermediate reaction configurations of dissolving Fe.