BACKGROUND Iron (Fe)-containing groundwater treatment residual is a kind of waste from groundwater treatment plants, showing good potential to be reused. Attention has been paid to Fe-based magnetic water treatment materials for their easy magnetic separation, and in the meantime, kinds of adsorbents were developed to address the severe global dyes pollution. RESULTS Black magnetic material was obtained with using the iron-containing wastes as iron source without adding other iron chemical reagents by hydrothermal reaction. The X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) results show that the product is comprised of crystalline iron(II,III) oxide (Fe3O4) core and amorphous carbon shell (Fe3O4@C), and the core-shell structure can also been seen clearly in transmission electron microscopy (TEM) images. Vibrating sample magnetometer (VSM) result shows its saturation magnetization is 33.56 emu/g. This sample's sorption experiments for methylene blue (MB) removal were conducted, and the experiment results indicated that this sample could be used in a wide range of pH (5-12) and the adsorption might be a complex chemical diffusion process consisting of boundary layer effect due to kinetic modeling fitting analysis. The maximum sorption amount of MB was 16.33 and 19.13 mg/g, respectively at 298 K and 308 K. This sample after five regeneration cycles still had 72.1% MB removal rate. CONCLUSION The Fe3O4@C particles with good magnetism were synthesized successfully with Fe-containing water treatment residuals, water treatment waste sludge, via a one-step hydrothermal reaction, which could be of reasonable analogy for treating other Fe-containing industrial waste. Furthermore, this sample could be used as adsorbent for MB molecules removal from dyed water with good reusability and easy magnetic separation. (c) 2019 Society of Chemical Industry