Olive stones have been widely used as a renewable energy biowaste source. As they are rich in elemental carbon (40–45 wt%), much research focussed on effectively converting olive stones, as precursors, into activated carbon adsorbents. However, only a few studies have concentrated on summarising the various techniques used to produce activated carbon from olive stone. This article reviews the research undertaken on the production and application of activated carbon as an adsorbent from olive stones for wastewater treatment. Various physical, chemical and physico-chemical treatments to remove heavy metals, organics and dyes are discussed, and the resultant adsorption capacities are reported. In several cases, very high adsorption capacities are recorded. Finally, the future prospects of these materials as adsorbents are discussed, and after further development work, olive stone-derived activated carbons have great potential especially in the area of organic polluted wastewaters. Activation process of olive stone Graphical abstract

Water pollution, from industry and agriculture, is a major threat to water security in the light of decreasing global potable water resources. Adsorption is one potential treatment technology for the removal of different contaminants from polluted water. Hard lignocellulose-containing materials with high carbon content, such as date stones, are excellent precursors for the production of activated carbons for adsorption activities. In this study, waste date stone biomass, from the seedless date products industry, was treated and activated chemically for the adsorption of malachite green dye, which is widely used in the plastics and textile industries. Phosphoric acid was used as an activating agent for the date stones, and its adsorption capability was measured and compared to the capacity of natural date stone powder. Experimental equilibrium data were analysed and correlated by four isotherm models: Langmuir, Freundlich, Temkin and Langmuir–Freundlich (also called SIPS). The best fit for the experimental data for both treated and natural date stone was the Langmuir–Freundlich isotherm. The surface areas and maximum malachite green adsorption capacities of the natural date stone and the phosphoric acid–treated date stone were 86 and 909 m²/g, respectively, and maximum malachite green adsorption capacities were 31.5 and 64.7 mg/g, respectively. A two-stage batch adsorber model was developed to optimize the two-step adsorption process to remove malachite green and minimize the amount of adsorbent used for both untreated and treated date stones. The benefits of the two-stage adsorber and chemically activated date stones are discussed in terms of designing for various percentages dye removal, various effluent discharge concentrations and various initial dye concentrations. The two-stage system resulted in a reduction of adsorbent compared with the one-stage system of 36 to 460% to remove the same amount of dye.