Methods for processing pretreated phosphogypsum wastewater are disclosed. The pretreated wastewater may be subjected to electrodialysis involving at least one monovalent cation selective membrane. Further downstream membrane treatment may be applied. Upstream precipitation and air-stripping techniques may optionally also be employed. Related systems are also disclosed.

In a method for recovering an active metal of a lithium secondary battery, a sulfuric acid solution is added to a lithium metal composite oxide so as to prepare a sulfated active material solution. A transition metal is extracted from the sulfated active material solution. A lithium precursor is recovered by adding a lithium extracting agent to the solution remaining after the transition metal has been extracted from the sulfated active material solution. In the method, the amount of impurities is reduced, and sulfuric acid and the neutralizing agent can be recycled so that a high-yield lithium precursor recovery is enabled.

A carbon dioxide electrolytic device includes: a carbon dioxide electrolysis cell having a cathode and an anode flow path, a cathode, an anode, and a first diaphragm; a first current regulator to supply a first current; a first gas/liquid separator to separate a first fluid from the anode flow path into a first liquid and gas; an electrodialysis cell having, first and second electrodes, first to fourth rooms, and second to fourth diaphragms; a second current regulator to supply a second current; at least one detector out of a first detector to detect a flow rate of the first gas or a concentration of carbon dioxide in the first gas, and a second detector to detect a pH or a concentration of at least one ion in the first fluid; and a first controller to regulate a second current, in accordance with at least one detection signal.

A method for separating and recovering carbon dioxide from ambient air includes continuously bringing ambient air into contact with a basic aqueous solution; electrodialysis of the solution using bipolar and anion-selective ion exchange membranes as well as recycling the depleted solution; separating CO.sub.2 from the enriched solution and recycling the solution depleted of CO.sub.2. The absorption is performed in an absorber, open basin, or a combination thereof. Separation is achieved by thermal desorption of CO.sub.2 by steam stripping to obtain a carbon dioxide/steam mixture; and/or by chemical reaction of the (hydrogen-) carbonate ions, in which the CO.sub.2 contained is converted into a water-insoluble salt or a gas and simultaneously removed from the solution. The pH of either obtained solution is measured before the recycling or before the separation, and is adjusted to a predetermined value. pH is measured and adjusted based on how absorption and separation are performed.

In a method for recovering an active metal of a lithium secondary battery, a sulfuric acid solution is added to a lithium metal composite oxide so as to prepare a sulfated active material solution. A transition metal is extracted from the sulfated active material solution. A lithium precursor is recovered by adding a lithium extracting agent to the solution remaining after the transition metal has been extracted from the sulfated active material solution. In the method, the amount of impurities is reduced, and sulfuric acid and the neutralizing agent can be recycled so that a high-yield lithium precursor recovery is enabled.