<![CDATA[Iron sand is an abundant natural resource in Indonesia. In this study, Fe3O4 was synthesized from iron sand originating from the Tekana River, OKU, South Sumatra, and applied for photocatalytic degradation of Congo red dye (CRD). Fe3O4 was synthesized using the coprecipitation method at pH variations of 9, 10, and 11. The characterization results showed that increasing pH resulted in better crystallinity. Fe3O4 synthesized at pH 11 had a cubic structure with the smallest crystal size of 28.4 nm and the largest saturation magnetization of 74.66 emu/g. The best Fe3O4 was then used for Congo red dye degradation and phytotoxicity testing. Photodegradation optimization was carried out by Response surface methodology (RSM) using Central composite design (CCD) through three independent variables: solution pH, Congo red concentration, and irradiation time. The quadratic model was identified as the most suitable model to describe the degradation process with optimum conditions at pH 4, concentration of 37.5 mg/L, and irradiation time of 62 min, resulting in a degradation efficiency of 98.93%. In addition, Fe3O4 showed good stability with a 6.91% decrease in efficiency after five reuse cycles. The results of phytotoxicity tests using Vigna radiata seeds showed no significant difference in plant growth using the degraded dye solution media and the control. These results indicate that Fe3O4 from iron sand is effective and stable as a photocatalyst, and also safe for the environment.Keywords: Iron sand, Fe3O4, Congo red dye, RSM optimization, phytotoxicity]]>
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