# A Novel Technosol Formulation for Sustainable Landfill Top Covers Using Non-Hazardous Wastes
## 3 Results and Discussion
### Vegetative Landfill Cap  
Following a thorough assessment of geotechnical properties, the blend 90 WFS/10 WAS has demonstrated compliance with requirements for compaction, bearing capacity, permeability, and shear strength, presenting a promising option for Technosol-based landfill capping applications.  
To promote a thriving vegetative layer conducive for plant growth on the landfill surface, the blend 90 WFS/10 WAS was enhanced with the addition of CBS at a ratio of 81 WFS/9 WAS/10 CBS. This amended formulation, comprising 81% waste foundry sand, 9% waste aggregate sludge, and 10% compost biosolids containing 54% organic matter (dry weight basis), was specifically tailored to attain a minimum of 5% organic matter content in the final product. The CBS was thoroughly integrated into the existing mixture to ensure uniformity, mimicking the homogenizing effects of plowing or tilling.  
Notably, the blend 81 WFS/9 WAS/10 CBS was crafted utilizing waste materials with physical-chemical parameters (pH, Eh, and electrical conductivity) closely resembling natural soils. Consequently, it can be expected that this uppermost layer of the Technosol not only facilities plant growth but also mitigates environmental risks associated with potential trace element leaching due to their low concentrations within the formulation.  
In fact, subsequent pot experiments using the formulation 81 WFS/9 WAS/10 CBS and planting _B. juncea_ yielded promising results. Within a week of planting, the plants displayed a remarkable ability to thrive within the Technosol, with shoots emerging as an indicator of the favorable conditions (Figure 7a). Over a 120-day period, four out of the five plants in each pot remained viable (Figure 7b), demonstrating the Technosol's capacity to support healthy vegetation growth, potentially leading to increased evapotranspiration, reduced water infiltration and leachate generation, and improved erosion control by stabilizing the landfill cover surface.  
The results of the leaching test conducted to assess the potential release of trace elements from the blend 81 WFS/9 WAS/10 CBS are presented graphically in Figure 8, along with pH, Eh, and electrical conductivity measurements. The recovered solutions showed a mildly alkaline pH (7.92), an oxidizing redox potential (Eh = 416 mV), and relatively low electrical conductivity (0.62 mS cm\({}^{-1}\)), suggesting limited concentration of water-soluble salts within the Technosol.  
The waste materials integrated into the blend 81 WFS/9 WAS/10 CBS contain significant amounts of acid-neutralizing components, such as calcite in WFS, and adsorbent phases, like iron oxy-hydroxides in WAS and organic matter in CBS. These components play a crucial role in limiting the mobility of potentially toxic trace elements, thereby, reducing the risk of environmental contamination [44; 45]. Indeed, the pool of trace elements available for plant uptake and leaching through percolation was found to be negligible. The most prevalent trace elements in the leachates, ranked in descending order of concentration, were Cu, As, Zn, and Pb, with mean values as low as (in \(\upmu\)g L\({}^{-1}\)) 19.29 Cu, 11.85 As, 10.01 Zn, 9.56 Pb, and 2.76 Ni. Other trace elements of concern, such as Cr, Se, Sb, and Cd, were detected at levels around or below 1.0 \(\upmu\)g L\({}^{-1}\).  
Figure 8: Trace element concentrations and physical-chemical parameters measured in the solution leached from the vegetative layer (81 WFS/9 WAS/10 CBS) of the Technosol.  
Figure 7: Representative growth stages of _Brassia junca_ within the vegetative layer (81 WFS/9 WAS/10 CBS) of the Technosol: (**a**) early seedling stage; and (**b**) pre-flowering stage.  
When compared to international water quality guidelines, the levels of trace elements released from the Technosol generally fell below safe drinking water standards. The exception was As, which slightly exceeded the highest permissible value of 10 ug L\({}^{-1}\) set by the EU Directive 98/83/CE and the World Health Organization for drinking water quality [46].