I would suggest that you check if citrate can indeed be considered a suitable sequestering agent for the considered element, at a given oxidation state, and if the resulting citrate complex is soluble in the considered lixiviate solution. For instance, you may sequester cobalt(II) using citrate, and cobaltous citrate can, in principle, be soluble in that solution. At a later step, you may want to break down the citrate complex (for demasking), possibly by adding a strong mineral acid, such as sulfuric acid: Co3(C6H5O7)2 (aq) + 3 H2SO4 (aq) → 3 CoSO4 (aq) + 2 C6H8O7 (aq). Cobaltous sulfate, in principle also soluble in the acidified solution, can be crystallised as heptahydrate (< 44 ºC) from previously concentrated solution, and recrystallised from water (up to three times) if required to improve its purity. Molybdenum(III) can also be complexed (sequestered) with citrate.

Dear Professor Carlos Araújo Queiroz, thank you for your reply.

Could you recommend some publications considering the mentioned subject?

Besides classic analytical chemistry textbooks that cover complexometry, the signalled book references also may help to identify suitable masking/demasking agents or procedures:

D.D. Perrin, "Masking and Demasking of Chemical Reactions: Theoretical Aspects and Practical Applications (Chemical Analysis)", Series: Chemical Analysis (vol. 33), Wiley-Interscience, New York, 1970.

Eugene W. Berg, "Physical and Chemical Methods of Separation", McGraw-Hill, New York (etc.), 1963, sections 18.3 & 18.4.