Building waste sludge as nutrient adsorbents for improving soil quality

Abstract

This study investigates the potential use of waste building sludge (WBS) as soil improvement additives after enrichment with nutritionally attractive ions (NH₄⁺ and PO₄³⁻ ). Two types of sludge were tested: (i) the concrete sludge (B) and, (ii) the sludge from the production of artificial stone (TS), both enriched with NH₄⁺ and PO₄³⁻ by adsorption from amodel solution. The enriched sludge (B_sorb. and TS_sorb.) were then incorporated into two soils with different characteristics: (i) sandy-loam soil (2.1), and (ii) clay-loam soil (6S). Subsequently, leaching experiments were performed to determine the release behavior of adsorbed ions and to assess the overall impact of the enriched sludges on the nutrient content of the soils. In the enrichment step, B showed a high adsorption efficiency of PO₄³⁻ (≈99 %), which mainly resulted from the precipitation of poorly soluble calcium phosphate rather than true adsorption. This indicates limited availability of PO₄³⁻ to plants. In contrast, TS released PO₄³⁻ into the solution and acted as a secondary nutrient source. In the case of NH₄⁺ enrichment, TS showed a higher NH₄⁺ adsorption efficiency (approaching 60 %) than B, where the high alkalinity limited NH₄⁺ adsorption. Leaching test showed that TS_sorb. increased PO₄³⁻ and NH+4 content in both soils, with higher effect in soil 2.1, whereas Bsorb. improved only 2.1 soil. Overall, the results show that enriched WBS, particularly TS_sorb., have potential to enhance the quality of nutrient-deficient soils.