Ferrous oxalate is most commonly encountered in its dihydrate form, i.e., FeC2O4·2H2O. The dihydrate of ferrous oxalate is a coordination polymer that consists of chains of oxalate-bridged ferrous centers, each capped by water molecules. When heated, it dehydrates and decomposes into iron oxides and a pyrophoric black iron, and it also evolves carbon dioxide and carbon monoxide gases. Ferrous oxalate is used for decorative glassware, as a pigment for plastics, paints, and lacquers, in the metal treatment industry, in photo developers’ formulations, and the textile industry.

Nucleation and crystal growth kinetics of ferrous oxalate

Ferrous oxalate is a simple metal organic framework co-ordinated polymer. Recently, ferrous oxalate has been extensively studied because of its applications in many fields. The synthesis plays an important role in uncovering their shape-dependent properties and fully achieving their potential practical applications. In spite of the fact that it is used in many fields, only few papers deal with the precipitation kinetics of ferrous oxalate. Abdel-Ghafar and Abdel-Aal studied nucleation aspects and morphology of iron (II) oxalate dihydrate crystals in water and diluted phosphoric acid media. But the nucleation rate expression was calculated according to the equation reported in a handbook. An improved apparatus is used for nucleation measurements according to Nielsen's method. A new method is proposed to calculate the dilution ratio N of the reaction solution during nucleation rate determination. With the rule, when the initial apparent supersaturation ratio S’=f(N) in the dilution tank is controlled from 1.3 to 3.0, crystal nucleus dissolving and secondary nucleation can be avoided satisfactorily. Experiments are realized by varying the supersaturation ratio from 15.6 to 93.3 and temperature from 15 °C to 50 °C. Ferrous oxalate is precipitated by mixing equal volumes of ferrous sulfate and oxalic acid solution.[1]

The experimental results showed that the nucleation rate of ferrous oxalate in the supersaturation range above is characterized by the primary homogeneous mechanism and can be expressed by the equation RN= ANexp(-Ea/RT)exp[-B/(ln S)2], where AN= 3.9×1013m?3s?1, Ea= 33.9 kJ mol?1, and B =13.7. The crystal growth rate can be expressed by equation G(t)=kgexp(-E'a/RT) (c-ceq)g, where kg= 3.6 × 1013m/s, E'a= 58.0 kJ mol?1, and g = 2.4.An improved apparatus is used for nucleation measurements according to Nielsen's method. And a new method is proposed to calculate the dilution ratio of the reaction solutions during nucleation rate determination. The method is successfully used here for the determination of nucleation rate expression of ferrous oxalate. With the rule, when the initial apparent supersaturation ratio S′ in the dilution tank is controlled from 1.3 to 3.0, crystal nucleus dissolving and secondary nucleation can be avoided satisfactorily. To simulate the continuous precipitation operations in the industry crystallizers, the ferrous oxalate crystal nucleus created in the tube during nucleation are used as the seed charge and the growth kinetics of ferrous oxalate is also obtained.

Ferrous oxalate mediated photo-Fenton system

This study assessed the applicability of a ferrous oxalate mediated photo-Fenton pretreatment for indigo-dyed wastewaters as to produce a biodegradable enough effluent, likely of being derived to conventional biological processes. The photochemical treatment was performed with ferrous oxalate and hydrogen peroxide in a Compound Parabolic Concentrator (CPC) under batch operation conditions. The reaction was studied at natural pH conditions (5–6) with indigo concentrations in the range of 6.67–33.33 mg L?1, using a fixed oxalate-to-iron mass ratio (C2O42?/Fe2+ = 35) and assessing the system's biodegradability at low (257 mg L?1) and high (1280 mg L?1) H2O2 concentrations. In order to seek the optimal conditions for the treatment of indigo dyed wastewaters, an experimental design consisting in a statistical surface response approach was carried out. This analysis revealed that the best removal efficiencies for Total Organic Carbon (TOC) were obtained for low peroxide doses. In general it was observed that after 20 kJ L?1, almost every treated effluent increased its biodegradability from a BOD5/COD value of 0.4. This increase in the biodegradability was confirmed by the presence of short chain carboxylic acids as intermediate products and by the mineralization of organic nitrogen into nitrate.[2]

The aim of this work is to evaluate the performance of ferrous oxalate type complexes to enhance the biodegradability of indigo-polluted wastewater at near neutral conditions (pH 5–6). These analyses will be carried out on simulated wastewaters, prepared according to modern dye-techniques. This part of the investigation was oriented to evaluate the reduction efficiencies of the TOC concentration according to a series of known factors that affect the performance of the photo-Fenton process mediated with ferrous oxalate such as the hydrogen peroxide ferrous ion and oxalate doses, the initial concentration of indigo and pH. Based on the present study on the treatment of a series of indigo-dyed simulated wastewaters by the solar photo-assisted Fenton process mediated with ferrous oxalates, the following conclusions can be drawn. The best treatment conditions found for indigo are the ones that generally follow low H2O2 concentrations, at an initial pH of 5–6. These conditions produce noticeable reductions in pollutant concentration, keeping the iron concentration low and, thus, economizing the reagent. The use of oxalate ion as a ligand of iron improves the performance of small iron concentrations at near neutral pH without precipitation of iron species.

Synthesis of submicron ferrous oxalate from red mud

Ferrous oxalate dihydrate (FOD) can be used as a photo-Fenton catalyst with remarkable photo-Fenton catalytic and photocatalytic performances on organic pollutant degradation. As an efficient heterogeneous photo-Fenton catalyst, FOD can activate hydrogen peroxide (H2O2) or persulfate and generate ROSs due to its high photosensitivity and self-activation capacity as C2O42? in the catalyst can reduce Fenton-generated Fe3+ to Fe2+, thus increasing the reaction rate. In addition, Ferrous oxalate dihydrate is also used as absorbents for the removal of dyes and heavy metals from the contaminated systems. Since HA-FOD exhibited the strongest photo-Fenton catalytic performance in comparison with the other two Ferrous oxalate dihydrate catalysts, a series of comparative experiments were carried out to obtain the optimal experimental parameters of HA-FOD on the degradation of MB. The effect of HA-FOD dosage on MB degradation was first investigated at the conditions of initial MB concentration of 50 mg/L, H2O2 dosage of 20 mg/L, and initial pH of 5.0.[3]

In summary, hydroxylamine hydrochloride was employed to reduce Fe3+ in soluble ferrioxalate complex solution leached from RM and could keep the system in a lower pH value via a hydrothermal process. HA-FOD was synthesized with a submicron size ranging from 100 nm to 1 μm with less impurity contents. This catalyst could effectively remove MB with improved photo-Fenton catalytic and photocatalytic capacities in comparison with the other two Ferrous oxalate dihydrate catalysts obtained under UV or natural light irradiation. The combined photocatalysis and photo-Fenton method could save more than 20% of the H2O2, lowering reagent cost in practical sewage treatment process.  This study provides a new method to synthesize ferrous oxalate from red mud with high Fenton catalytic performance and also provides a perspective on utilizing iron-rich industrial waste to prepare HA-FOD mineral clusters to remediate organically polluted wastewater through the novel synthetic method.

References

[1]Li C, Ning Y, Yan T, Zheng W. Studies on nucleation and crystal growth kinetics of ferrous oxalate. Heliyon. 2019 Nov 19;5(11):e02758. doi: 10.1016/j.heliyon.2019.e02758. PMID: 31768433; PMCID: PMC6872798.

[2]Vedrenne M, Vasquez-Medrano R, Prato-Garcia D, Frontana-Uribe BA, Hernandez-Esparza M, de Andrés JM. A ferrous oxalate mediated photo-Fenton system: toward an increased biodegradability of indigo dyed wastewaters. J Hazard Mater. 2012 Dec;243:292-301. doi: 10.1016/j.jhazmat.2012.10.032. Epub 2012 Oct 22. PMID: 23142056.

[3]Yang Y, Wang N, Gu H. Synthesis of submicron ferrous oxalate from red mud with high Fenton catalytic performance on degradation of methylene blue. Environ Sci Pollut Res Int. 2023 Aug;30(36):85210-85222. doi: 10.1007/s11356-023-28308-z. Epub 2023 Jun 29. PMID: 37386219.