The effect of adding 10% biochar (B) or sludgechar (SL) on the water regime and adsorption properties of soils was tested on composites prepared by mixing two standard soils of loamy and clay type with B or SL in a 90:10 weight ratio. Water-holding capacity was assessed as initial (2 h) and equilibrium (24 h). Water retention time was estimated by evaporation from saturated samples at 20 °C to a constant weight. The composites exhibited a 60–90% increase in water absorption compared to the individual soils, retaining water up to 3–6 days longer than the individual soils. The adsorption properties were tested for cation (Pb²⁺) and anion (Sb(OH)₆⁻) adsorption and for Pb²⁺ and Sb(OH)₆⁻ co-adsorption from model solutions under laboratory conditions. All samples showed higher selectivity for Pb²⁺, with the adsorption efficiency from 40% to 99%. Sb(OH)₆⁻ adsorption achieved a maximum efficiency of only 10%. Pb²⁺ and Sb(OH)₆⁻ co-adsorptions were efficient for Sb(OH)₆⁻ adsorption, reaching efficiency levels above 95%. At prolonged reaction times, the adsorption efficiency elevated by more than 20%. Only 10% wt. addition of biochar or sludgechar enhanced not only the water regime of soils but also their adsorption capacity for ionic contaminants.

The expected benefit of composite adsorbents generally consists in their growing applicability, thanks to the combination of the adsorption properties of individual components. Composite adsorbents were prepared as mixtures of kaolin from a Czech deposit (kaolin Sedlec, SK) and two fly ashes (FAs) from a fluidised bed boiler in Czech operations differing in fuel type. The mixtures of SK with FA in a ratio of 50:50% wt. were prepared at 20 °C, 65 °C, and 110 °C in an autoclave. The source materials and composite adsorbents were tested for the adsorption of lead as Pb²⁺, and arsenic as AsO₄³⁻ from model solutions in laboratory conditions. The adsorption of Pb²⁺ proceeded quantitatively on the source materials except SK, and on both the composites, with an adsorption yield of >97% and a low adsorbent consumption (~2 g.L⁻¹). The AsO₄³⁻ adsorption proceeded selectively only on both FAs, with an adsorption yield of >97% again. The adsorption of AsO₄³⁻ on the composite adsorbents achieved a worse yield (˂80%), with about ten times more adsorbent consumption (~20 g.L⁻¹). An increased preparation temperature did not affect the Pb²⁺ adsorption at all, but it reduced the efficiency of AsO₄³⁻ adsorption by up to 30%. The SK–FA composites proved to have promising properties, mostly as cation-active adsorbents.

Two selected waste building sludges (WBS) were used in this study: (i) sludge from the production and processing of prestressed concrete pillars (B) and (ii) sludge from the production of technical stone (TS). The materials were used in their original and Fe-modified forms (B_Fe/TS_Fe) for the adsorption of NH₄⁺ and PO₄³⁻ from contaminated waters. The experiments were performed on a model solution simulating real wastewater with a concentration of 1.7 mmol·L⁻¹ (NH₄⁺) and 0.2 mmol·L⁻¹ (PO₄³⁻). The adsorption of PO₄³⁻ had a high efficiency (>99%) on B, B_Fe and TS_Fe, while for TS, the adsorption of PO₄³⁻ was futile due to the high content of available P in the raw TS. The adsorption of NH₄⁺ on all sorbents (B/B_Fe, TS/TS_Fe) had a lower efficiency (<60%), while TS proved to be the most effective. Leaching tests were performed according to the CSN EN 12457 standard for B/B_Fe and TS/TS_Fe before and after NH₄⁺ and PO₄³⁻ sorption when the contents of these ions in the leachates were affected by adsorption experiments in the cases of B and TS. For B_Fe and TS_Fe, the ion content in the leachates before and after the adsorption experiments was similar.