Addressing water pollution issues requires innovative solutions and global cooperation for sustainable water resources. In this study, the design of calcium ferrite nanoparticles was optimised by investigating different Ca:Fe molar ratios, calcination temperatures, and synthesis methods: thermal decomposition (TD) and sol-gel combustion (SG). We evaluated the magnetic behaviour, the photocatalytic degradation of cationic/anionic dyes (methylene blue, MB; methyl orange, MO) and the adsorption of phosphates (PO₄3−). We found that only SG samples exhibited superparamagnetic behaviour, with a high value of maximum magnetisation (5–40 Am2 kg Fe−1). TD samples showed superior photocatalytic activities, up to 4.5 mg g−1 for MB and 2.0 mg g−1 for MO. The phosphate adsorption capacity of both TD and SG CaFe_1:1 samples was very promising, with values exceeding 100 mg g−1 for TD samples and ranging between 50 and 100 mg g−1 for SG samples. We systematically investigated the structural and colloidal properties to identify which design parameters most significantly affected the functional ones. Photocatalytic activity and phosphate adsorption capacity were found to be strongly dependent on the crystal phase of the samples. The samples exhibiting the highest activity were those containing brownmillerite (Ca2Fe2O5), followed by those containing hematite. The orthorhombic off-spinel phase (O-CaFe2O4) did not exhibit significant activity unless brownmillerite or hematite crystalline phases were also present. Finally, only SG samples Ca:Fe 1:2_300-750 in the presence of both cubic spinel phase (C-CaFe2O4) and brownmillerite showed relevant magnetic and photocatalytic activities, with SG sample Ca:Fe_1:2_500 exhibiting the highest photocatalytic efficiency. The other physicochemical properties had little influence on the reactivity of the investigated ferrite phases, including specific surface area, which was significantly higher for the SG samples compared to the TD samples.