Investigation of biomass waste biochar production to act as matrix for urea

Abstract

In this work, the synthesis of biochar from several biomass wastes to act as matrix for urea was investigated. The objective was to select the most promising biochar synthesis condition that result in a matrix with the highest urea retention and subsequent slow release as well as to understand the urea–biochar interaction. A quite simple urea impregnation method was proposed and investigated for biuret formation; its experimental conditions were optimized to increase final urea retention performance. The results pointed out that the urea–biochar interaction has chemical nature, in which the lower presence of mineral elements associated with an acid surface seemed to favor the urea retention. The material impregnated at 133 °C for 1 h with the straw biochar synthesized at 400 °C for 0.5 h showed the lowest initial release of urea (≈ 17 wt. %). However, it presented a high content of biuret (≈ 8 wt. %). Thus, an alternative material was produced applying a shorter period of urea impregnation (10 min); it also maintained the initial release of urea ≈ 17 wt. %.

Appendices

Appendix A

Below, it can find the formulas used in the quantification techniques of this work.

Pyrolysis yield

$$Yield \left(\mathrm{\%}\right)={m}_{f}/{m}_{0} \times 100$$

(A.1)

In which: m₀—initial biomass mass (g);

m_f—biochar final mass (g).

Percentual of the initial release

$$\mathrm{\% }Initial release={m}_{u0}/{m}_{ut} \times 100$$

(A.2)

In which: m_u0—urea mass quantified in the volume of liquid recovered at the first monitoring point (t = 20 min);

m_ut—total mass of urea in the release agent (t = 0).

Volatile matter (vm)

$$vm\left(\mathrm{\%}\right)= ({m}_{d}-{m}_{h})/{m}_{d}$$

(A.3)

In which: m_d—the mass of dried sample (g);

m_h—mass post-heating (g).

Ash content (ac)

$$ac (\mathrm{\%})={m}_{a}/{m}_{d} \times 100$$

(A.4)

In which: m_a—ash mass after the final heating (g);

m_d—the mass of dried sample (g).

Boehm titration

$${n}_{CSF}= {[V}_{T}{N}_{b}\left({V}_{am}-{V}_{b}\right)]/({V}_{al}{m}_{bu})$$

(A.5)

In which:

n_csf—the amount of acid functional groups on the surface of the solid that reacted with the base (mol.g⁻¹);

N_b—concentration of the titrant (mol dm^{−3 −1});

V_am—Volume of titrant spent with the sample (dm⁻³);

V_b—Volume of titrant spent on blank titrations (dm⁻³);

V_al—Filtrate aliquot volume (dm⁻³);

V_T—Volume of solution added to the coal (dm⁻³);

m_bu—the mass of previously dried biochar (g) (Table 3).

In the calculation of alkaline groups, the order of subtraction was inverted from (V_am—V_b) to (V_b—V_am).

Appendix B

Table 3 Urea release performance parameters of sample group I