Introduction As a bulk solid waste in iron and steel industry, steel slag occupies a large amount of land, and causes environmental pollution, having a great burden to iron and steel industry and society. It is thus of great theoretical and practical significance to explore effective ways of steel slag recycling and utilization. Formaldehyde (HCHO) as a colorless, irritating smell of toxic gas is one of common volatile organic compounds (VOCs) in indoor air, which can cause chronic respiratory diseases, leukemia, and bronchial asthma and damage to the nervous system. Activated carbon (AC) is extensively used as an adsorbent for removal of HCHO. AC is an extraordinary adsorbent, but its adsorption efficiency for HCHO is low. The modification of AC using metal oxides (such as V2O5, MnO2, CuO, Fe2O3, etc.) can improve the service life of AC and its formaldehyde adsorption performance. However, the preparation cost will increase due to the high price of metal oxides. Steel slag is rich in oxides of calcium, silicon, magnesium, iron, manganese, phosphorus and other elements. Therefore, the composite preparation of ecological AC by steel slag and biomass waste materials solves the problems of high modification cost, short service life and poor absorptivity, and expands a way of high value-added utilization of steel slag.Methods Steel slag/peanut shell activated carbon (SPAC) was firstly prepared via microwave heating and phosphoric acid activation. According to the national standard of “Formaldehyde emission limit in wood-based panels and their Products of Interior Decoration Materials” (GB18580-2017), a formaldehyde absorptivity experiment by SPAC was systematically made. The porosity of samples was determined by a model ASAP 2020M surface area & porosity analyzer based on the N2 adsorption-desorption isotherm. The microscopic morphology of SPAC was analyzed by a model NANO SEM430 scanning electron microscope with energy disperse spectroscope (SEM-EDS). The elements content of SPAC in ultra-high vacuum with Al Kα laser radiation (hυ=1 486.6 eV) was characterized by a model ESCALAB250 X-ray photoelectron spectrometer (XPS).Results and discussion The result show that the sample has an excellent removal efficiency (i.e., 93.2%) under the preparation condition of 550 W microwave power, 1.25 impregnation ratio and 15% steel slag content. The correlation coefficients of the experimental results and the fitted values for the pseudo-second-order kinetic model and the Freundlich model are 0.998 0 and 0.953 7, respectively. It is indicated that the removal process of formaldehyde on SPAC is more dominant due to chemisorption. The N2 adsorption-desorption isotherm is a type IV adsorption isotherm with H3 and H4 hysteresis loops, indicating that the sample is a mesoporous material. Based on the results of SEM-EDS and XPS, steel slag is loaded into the SPAC sample and elements Fe and Mn have a crucial role for the absorptivity of formaldehyde. This study provided a reference for the exploration of steel slag based functional materials, and a theoretical support for the efficient formaldehyde removal at room temperature.Conclusions Steel slag powder could be loaded into the composite activated carbon. The prepared composite activated carbon still retained the porous structure characteristics suitable for removal of formaldehyde. The synergistic action of the elements Fe and Mn from steel slag powder was beneficial to improving the formaldehyde removal.