# Biomass Fly Ash Self-Hardened Adsorbent Monoliths for Methylene Blue Removal from Aqueous Solutions
## 3 Results and Discussion
### MB Adsorption Tests
#### 3.3.5 Comparison with Other Studies  
Table 6 compares the obtained \(q_{max}\) values with several ones reported in the literature for coal fly ash (CFA) and BFA. Most studies used powdered samples and \(q_{max}\) values varied between \(\sim\)1 and \(\sim\)11 mg/g, with the minimum value corresponding to trials with NaOH hydrothermally treated CFA. Powdered adsorbents have a much higher concentration of active sites, when compared to porous monoliths, due to their much higher surface area. However, once exhausted, the removal from the aqueous solutions of monoliths is much easier and practical unless the powder is inserted into a container. The \(q_{max}\) value (0.62 mg/g) obtained in this study is smaller than the one achieved with BFA-based geopolymer cylindrical monoliths (2.40 mg/g), which can be explained by the higher and more connected porosity in the latter. However, the processing of the monoliths developed in the present work is more sustainable and cheaper since it only uses BFA, water and a very small amount of Al powder. On the contrary, in the production of the geopolymeric samples, metkaolin was added to BFA, and hardening requires the use of alkaline activators (NaOH and sodium silicate), penalizing the cost and the environmental footprint.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Adsorbent Shape** & \(q_{max}\) **(mg/g)** & **Ref.** \\ \hline CFA NaOH hydrothermally treated & Powder & 10.82 & [23] \\ BFA H\({}_{2}\)O\({}_{2}\) heat-treated and sieved & Powder & 6.46 & [3;24] \\ CFA grounded & Powder & 6.04 & [3;13] \\ CFA washed & Powder & 5.72 & [16] \\ CFA unmodified & Powder & 2.97 & [25] \\ CFA microwave heated in HCl solution & Powder & 2.88 & [18] \\ CFA sonicated in HCl solution & Powder & 2.58 & [25] \\ BFA-based geopolymer & Cylindrical monolith & 2.40 1 & [4] \\ CFA ground and sieved & Powder (\(<\)90 \(\upmu\)m) & 1.47 & [3;13] \\ CFA & powder & 1.11 & [3;23] \\ CFA based geopolymer & powder & 0.67 & [2;26] \\
**BFA based self-hardened** & **Parallelepiped monolith** & **0.62** & **This work** \\ \hline \hline \end{tabular} \({}^{\dagger}\) Value obtained for the sample with the higher BFA content before regeneration steps.  
\end{table}
Table 6: MB adsorption capacity (\(q_{max}\)) of various fly ash adsorbents.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{\(\boldsymbol{C_{0}}\) **(ppm)**} & \multirow{2}{*}{\(\boldsymbol{q_{e}}\) _exp_ (mg/g)**} & \multicolumn{2}{c}{**Pseudo-First-Order**} & \multicolumn{2}{c}{**Pseudo-Second-Order**} \\ \cline{3-8}  & & & \(q_{e}\) **(mg/g)** & \(\boldsymbol{K_{1}}\) **(1/min)** & \(\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{ \boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{\boldsymbol{1}}}}}}}}}} }}}\) & **R\({}_{1}\)2** & \(q_{e}\) **(mg/g)** & \(\boldsymbol{K_{2}}\) **(g/mg min)**  
Finally, considering the further environmental management of the exhausted monoliths containing MB, they contain a known (but small) dye concentration and can be incorporated in mortar or concrete formulations, acting as light aggregate. If properly managed, dye molecules will remain immobilised in the new matrix and a solution with decreased waste generation might be achieved. In the literature, several works report the valorisation of BFA in cement-based mortars [27, 28, 29], as filler or aggregate. Nevertheless, this topic of environmental management of the monoliths produced will be addressed in future work.