For the adsorption of malachite green (MG), 'purified by thermo-acid (HCl) diatomite' (D-TA) and its derivative '3-mercaptopropyltrimethoxysilane functionalised diatomite' (DTA-MTMS) were prepared as adsorbent and the effects of thermo-acid activation and surface functional groups on adsorption were investigated. Many acids dissolve certain substances in diatomite and change their chemical composition. Thermo-acid treatment and silane-modified diatomite fractions were characterised byBrunauer-Emmett-Teller (BET), scanning electron microscope (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and thermal analysis (TG-DTA). The effects of contact time, solution pH, temperature and initial dye concentration on adsorption were investigated. Equilibrium isotherms are defined using nonlinear Langmuir, Freundlich, Dubinin - Radushkevich (D - R) and Sips adsorption isotherm equations. Isotherm data for both adsorbents are best fitted on the Freundlich and Sips model. The q(max) values determined from the Sips model are 18.8415 mgg(-1) and 21.4713 mgg(-1) for D-TA and DTA-MTMS, respectively. It was determined that DTA-MTMS performed better than D-TA for MG adsorption. The results showed that MG adsorption was compatible with the pseudo-second-order (PSO) kinetic model. From the adsorption energy value (E < 8 kJ mol(-1)), it was confirmed that physical adsorption was dominant in the adsorption process. The nature of the sorption process is exothermic and spontaneous (due to negative values Delta H degrees and Delta G degrees). The results of the study showed that prepared D-TA and synthesised DTA-MTMS can be considered as an economical and promising adsorbent in removing organic dyes from wastewater.