Biochar's remarkable, huge internal surface areas boost soil Cation Exchange Capacity (CEC) to retain positive ions like Calcium, Sodium, Magnesium, Potassium, etc.  But biochar has Anion Exchange Capacity (AEC) to capture negative ions, beginning with Nitrogen and Phosphorus ( N-P of N-P-K ).

Fresh biochars can have net positive or net negative surface charge, but typically have initially low cation exchange capacities (CEC) compared to soil organic matter on a mass basis . Notable is the initially measurable anion exchange capacity which disappears over time in soil , and in some cases strong interaction with phosphates . High-ash biomass generates biochars with slightly greater CEC and charge density upon normalizationof CEC to surface area (Table 1). On the other hand, greater

pyrolysis temperatures cause a decrease in CEC, especially in chargedensity as a result of the greater surface area produced at high temperatures of up to 600 C and loss of volatile matter , which may contain a substantial portion of the negative charge and CEC as organic acids. Sorption of comparatively polar organic matter

such as catechol or humic acid extracts to wood biochars increased in the range from 400 to 650 C, likely due to greater nanopore surface area. At even higher temperatures of up to 1000 C (typically to produce so-called “activated carbon”, but also gasification biochars), carbons are mainly hydrophobic and do not sorb appreciable amounts of nutrients or polar organic substances, such as sugars. The same has also been observed for naturally occurring black C . Rather, such high-temperature carbons sorb mainly non-polar or weakly polar organic solutes, notably those bearing aromatic structures, as have been described in a large number of studies in the scientific literature . These also include enzymes  and other substances important for microbial processes in soil. This body of information is not fully transferable to biochars that are produced at much lower temperatures, and without the addition of chemicals that are commonly used to increase the surface area of activated carbons. Nonetheless, prior research on activated carbon may be informative for describing the properties of biochars produced at comparatively high temperatures such as during some gasification procedures and may even apply for understanding some surface properties of lowtemperature biochars. In soil, biochars (those produced at or below 600e700 C) seem to oxidize rapidly and attain greater amounts of CEC , but initially still retain a significant proportion of non-polar surfaces . Source : Excellent review entitled Biochar effects on soil biota .  A review by Johannes Lehmann ..PDF enclosed for further reading..

http://www.biochar-international.org/biochar/faqs

Maybe this can help you a little bit.

The FAQ's are interesting as I've needed clarification as well. Biochar is a nice name for charcoal, plain and simple. It differs from charcoal only by definition of its intended use. Production of biochar still involves combustion of suitable biomass under controlled conditions which means it is a contributor to greenhouse gasses. Are there publications documenting research on P (and various N forms) adsorption?

Biochar has negative charge due to the presence of more -OH functional group and is closely related with pH. As the pH increases, the magnitude of negative charge also increases due to increased deprotonation of the functional group at elevated pH. That's why this material is mostly used to adsorb positively charged metals(iods).

Biochar's remarkable, huge internal surface areas boost soil Cation Exchange Capacity (CEC) to retain positive ions like Calcium, Sodium, Magnesium, Potassium, etc.  But biochar has Anion Exchange Capacity (AEC) to capture negative ions, beginning with Nitrogen and Phosphorus ( N-P of N-P-K ).

Fresh biochars can have net positive or net negative surface charge, but typically have initially low cation exchange capacities (CEC) compared to soil organic matter on a mass basis . Notable is the initially measurable anion exchange capacity which disappears over time in soil , and in some cases strong interaction with phosphates . High-ash biomass generates biochars with slightly greater CEC and charge density upon normalizationof CEC to surface area (Table 1). On the other hand, greater

pyrolysis temperatures cause a decrease in CEC, especially in chargedensity as a result of the greater surface area produced at high temperatures of up to 600 C and loss of volatile matter , which may contain a substantial portion of the negative charge and CEC as organic acids. Sorption of comparatively polar organic matter

such as catechol or humic acid extracts to wood biochars increased in the range from 400 to 650 C, likely due to greater nanopore surface area. At even higher temperatures of up to 1000 C (typically to produce so-called “activated carbon”, but also gasification biochars), carbons are mainly hydrophobic and do not sorb appreciable amounts of nutrients or polar organic substances, such as sugars. The same has also been observed for naturally occurring black C . Rather, such high-temperature carbons sorb mainly non-polar or weakly polar organic solutes, notably those bearing aromatic structures, as have been described in a large number of studies in the scientific literature . These also include enzymes  and other substances important for microbial processes in soil. This body of information is not fully transferable to biochars that are produced at much lower temperatures, and without the addition of chemicals that are commonly used to increase the surface area of activated carbons. Nonetheless, prior research on activated carbon may be informative for describing the properties of biochars produced at comparatively high temperatures such as during some gasification procedures and may even apply for understanding some surface properties of lowtemperature biochars. In soil, biochars (those produced at or below 600e700 C) seem to oxidize rapidly and attain greater amounts of CEC , but initially still retain a significant proportion of non-polar surfaces . Source : Excellent review entitled Biochar effects on soil biota .  A review by Johannes Lehmann ..PDF enclosed for further reading..

Thanks to all for your nice comments and suggestions.

The feedstock for biochar usually contains cations, e.g. Ca and Fe as well as silica and phosphates.  That is, biochar it is not pure charcoal and many of its properties relate to the presence of the "contaminants".  The surface charge will not be uniformly distributed.

Surface charge characteristics of biochar

Surface charge characteristics of soil and soil–biochar mixtures were assessed using the ‘index’ or ‘indifferent’ ion adsorption method with a LiCl electrolyte (Chorover et al. 2004). The method involved three main steps, as briefly described below:

1. Soil–biochar slurries, saturated with decreasing concentrations of LiCl (1, 0.2, and 0.01 M) were prepared using a soil : solution ratio of 1 : 5.

2. Approximately 0.3 g of Oxisol–biochar slurry and 0.8 g of Inceptisol–biochar slurry, on a dry-weight basis, were transferred to pre-weighed 50-mL centrifuge tubes; 20 mL of 0.01 M LiCl was added to the tubes and the solution pH was adjusted across a pH range of 3–7 in increments of 0.5. Equilibrium pH was measured immediately following centrifugation and the concentrations of Li and Cl in the supernatant were measured.

3. Ammonium nitrate (20 mL, 1 M) was added to the centrifuge tubes and shaken for 1 h to displace the Li and Cl ionsadsorbed by the soil or soil–biochar mixtures. The procedure was repeated twice and the extracts were pooled. The concentration of Li and Cl ions extracted by 1 M ammonium nitrate solution was measured using a Varian SpectrAA 220FS atomic absorption spectrometer (AAS) (Agilent Technologies Inc., Santa Clara, CA, USA) and a Metrohm Tiamo 1.3 ion analyser (Metrohm AG, Herisau, Switzerland), respectively....PDF enclosed for reference 

Source ; Soil Research, 2014, 52, 155–163,http://dx.doi.org/10.1071/SR13187

Biochar retains nutrients in soil directly through the negative charge that develops on its surfaces, and this negative charge can buffer acidity in the soil, as does organic matter in general.

One way to determine the surface charge of the Biochar is to determine its surface charge (zeta potential) as a function of pH. In this way knowing the pH can determine if its load is negative or positive.

Acid washing of low temp biochar may help the protonation of surface which can develop positive charge on biochar