Nutrient recovery (phosphorus and nitrogen) by adsorption to hydrochars, biochars and chemically modified biochars.

Adsorption of P on biochars and hydrochars is mainly related to the presence of Ca or Mg. Chemical modification of the chars can increase the adsorption capacity. The adsorbed P is not however easily desorbed, so possibly limiting usefulness of the products as fertilisers.

Biochars and hydrochars are materials produced by the thermochemical or hydrothermal treatment of organic matter. They can be obtained from a variety of waste biomass and have shown potential for recovering nutrients from various waste streams. Hence they are increasingly gaining attention as economical and environmentally sustainable products that can be applied in waste management.

Takaya et al studied at lab scale (0.1g of char used in experiments) sorption of phosphate (PO43-) and ammonium (NH4+) onto biochar and hydrochars obtained from a variety of waste raw materials (oak wood, greenhouse waste, anaerobically digested waste, treated municipal waste). The study aimed to (1) investigate the recovery potential for the nutrients (phosphate and ammonium) and (2) to understand the physicochemical properties (like elemental composition, mineral content, and surface functionality) that enhance the nutrient uptake on the biochars and hydrochars.

No significant differences between biochars
Cation exchange capacity (CEC) was measured as an important parameter that could be related to the surface functionality and surface area. A higher CEC would usually imply an increase in surface functionality, which could mean an improvement in nutrient uptake.  In the current study, no direct correlation between CEC and surface area was found. The variation of CEC as a function of pyrolysis temperature and treatment with solvent (toluene) was checked. It was observed that higher pyrolysis temperature (600 to 650 °C) generally resulted in higher CEC on the biochars than the lower temperature (400 to 450 °C). Solvent treatment generally resulted in higher CEC for hydrochar whereas for biochars the CEC remained unaffected or was lowered.

The phosphate and ammonium adsorption capacities for the materials ranged between 0 to 10 mg P/g and 80 to 115 mg N/g, respectively. The phosphate adsorption capacities for the biochars increased with pyrolysis temperatures, and the authors found positive correlation with Ca and Mg contents.

The authors therefore attribute the phosphate sorption capacities to metal ion reactions, which include precipitation and surface deposition. For ammonium, there was a positive correlation between the functional groups and CEC. The ammonium adsorption was mostly attributed to chemical adsorption with oxygen containing functional groups. However, the authors conclude that despite differences in physicochemical properties and processing conditions, there were no significant differences between the ammonium and phosphate adsorption capacities between the various chars.

Chemically modified biochars
In another study, Takaya et al. studied the recovery of phosphate with chemically modified biochars. The treatments included chemical activation with iron and magnesium salts, surface activations with potassium hydroxide (KOH) or hydrogen peroxide (H2O2). Biochars treated with magnesium salts gained a significant enhancement on phosphate uptake while modification with other chemicals resulted in marginal improvements on phosphate uptake. The biochars modified with magnesium salts exhibited a phosphate adsorption capacity of c. 50 mg P/g, which was much higher than other chars. In both the studies, desorption experiments were carried out with 0.01 M potassium chloride (KCl) solutions. The phosphate desorbed from the chars was low in most cases. The concentrations of phosphate used in both studies (usually greater than 125 mg P/L) were unrealistic for adsorption from effluent of a municipal wastewater plant. But the authors reason that such high concentrations can be found in other sources, such as in anaerobic digestion plants and in agricultural and industrial wastewaters.

Obstacles to use of biochars for nutrient recycling
It is not very surprising that the authors are not able to significantly desorb phosphate from the char, because adsorption of phosphate is usually strong enough that weak electrolyte solutions (like 0.01 M KCl) will not be enough to reverse the reaction and release phosphate. Especially, if phosphate adsorption occurs via chemisorption or even precipitation, strong conditions like high concentration of sodium hydroxide (pH >12) will be required to desorb the phosphate. Although chemically modified biochars are able to remove nutrient like phosphate form wastewater streams, to justify the term recovery, the adsorbed nutrients also need to be plant available. The results of this study show that the adsorbed P is not easily desorbed, but in soils other processes may play a role. This aspect was not part of the scope of this study.

References
Takaya, C. A., et al.
"Phosphate and ammonium sorption capacity of biochar and hydrochar from different wastes." Chemosphere 145 (2016): 518-527
http://dx.doi.org/10.1016/j.chemosphere.2015.11.052

Takaya, C. A., et al. "Recovery of phosphate with chemically modified biochars." Journal of Environmental Chemical Engineering 4.1 (2016): 1156-1165
http://dx.doi.org/10.1016/j.jece.2016.01.011

C. Takaya, L. Fletcher, S. Singh, U. Okwuosa, A. Ross, K. Anyikude, University of Leeds, LS2 9JT, United Kingdom

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