Petcoke as a soil for plant growth: comparison with a sandy soil

By: Athenas Fermin, Myloa Morgado, Maria Tosta, and Jorge Laine

Centro de Química, Instituto Venezolano de Investigaciones Científicas, Caracas, Venezuela.

Email: jlaine@ivic.gob.ve

ABSTRACT

Present short communication is part of a research dealing with possible use of fossil hydrocarbon coke for agricultural and desert greening applications. Germination and growth of sunflower plant were carried out using two types of soil: one was petroleum coke (petcoke); the other was sand having nil organic matter. Applying fertilizer irrigation, seeds started germination similarly in both soils, however some of the sunflower plants developed completely in the petcoke, whereas in the sand all the sprouts died before flowering. It is suggested that adequateness of NPK fertilizer compounds occurring on the petcoke surface is responsible for the observed difference.

Keywords: Petroleum coke, desert greening, sunflower, plant growth

Introduction

Desert greening is a matter concerning both agroforestry production and climate change. Two conditions are necessary to accomplish desert greening: one is to bring in proper soil adequateness for rizosphere fertility, and the other is to ensure water supply for irrigation.

In connection with the first condition, one option proposed is the land applications of biochar [1], a pyrogenic carbon derived from biomass. This application is based on the agricultural technique “slash-and-char”, probably employed by ancient Amazonian communities that created a soil, referred to as terra-preta (“black earth” in Portuguese), characterized by having very high organic carbon content and sustainable fertility. Indeed, because of organic chemicals affinities, sequestering pyrogenic carbon in soil improve soil fertility by preventing labile organic matter (particularly humus) from being rapidly mineralized and/or lixiviated.

Another form of pyrogenic carbon is coke, obtained from fossil hydrocarbons, which possible application for substituting biochar has been published elsewhere [2]; suggesting a model of the required chemical structure for soil fertility, consisting of a dual porous structure: one structure formed by slit-shaped small pores (near 10−9 m width) featuring substituted graphene sheets containing nutrient elements (N, P, K, etc.) as depicted in Figure 1, and another structure formed by large pores (near 10−6 m width) providing the

space for the habitat of plant friendly microorganisms such as rhizobium bacteria and mycorrhiza spores.

Besides of improving soil fertility, another effect of the application of pyrogenic carbon is that it makes soil darker. As sun light terrestrial reflection (i.e., the terrestrial albedo) is smaller in darker lands, inter-phase heat transfer between land and the atmosphere must be affected favoring exothermic changes such as cloud condensation. Undoubtedly, mean annual rainfall at non-permafrost latitudes is known to increase when terrestrial albedo decreases, as referred to elsewhere [3].

Accordingly, application of a pyrogenic carbon such as petcoke in soil may not only improve soil fertility, but also may improve rainfall for irrigation, the two necessary conditions cited above for desert greening.

Whiting the above scope, present short communication shows results on the germination and growth of sunflower plant, indicating that petcoke soil has better adequateness for rizosphere fertility compared with a sandy soil taken as a desert representation.

Experimental

Two artificial soils were employed: namely petcoke soil and sandy soil. The petcoke was obtained from refinery located nearby Orinoco´s heavy oil exploitation at Venezuela; and the sand (probably having nil organic matter) was one from a subsoil mine, employed by local builders for preparing Portland concrete. The petcoke was sparsely placed over a roof and submitted to open sky during about one year (Latitude: 10.5o N) before carrying seed sowing. This procedure was intended to eliminate most organic volatile matter, supposedly not suitable for rizosphere life. Another petcoke sample of same origin was not submitted to open sky but stored in close container for comparative purposes.

Seeds of sunflower (dwarf type, Pacino variety), same employed in preliminary experiments reported elsewhere [2], were individually sowed in separated pots of about 60 cm3 each, using 24 pots of each artificial soil. One gram of soluble fertilizer (NPK: 20-20-20) was dissolved in one liter of water for daily irrigation.

Results and Discussion

Germination and growth was carried open to sky all time during dry season (March until May 2014). After one week of sowing, about 50 % of the seeds sprout in each soil, but after about one month all sprouts died on the sand, but nine did survive in petcoke. Four seedlings were transplanted to a wider common container having a larger amount of the petcoke, where all bloom after about two months of sowing (see supplementary material).

XRD spectra shown in Figure 2 suggest certain degree of graphitization of petcoke and, in the case of sand, a very sharp peak corresponding to quartz. Estimating crystallite size with Scherrer formula using peak widths at peak half height indicated sand has quartz crystallites some hundred times larger than the petcoke graphite crystallites. This difference is in agreement with microscopy (Figure 3) where it can be noticed that petcoke particles have significant more surface roughness than sand particles. Petcoke sample kept in close container had a similar surface texture than that submitted to open sky.

EDAX semi-quantitative analysis (Table I), indicate that sand particle surface contains carbon, probably corresponding to non-crystalline inorganic carbonates concentrated at the surface rather than inside the crystalline bulk, which must be mainly quartz. In contrast to the petcoke kept in close container, the presence of significant oxygen in the petcoke submitted to open sky suggests surface oxidation after long exposure to ambient. Such oxidation probably created surface carboxylic sites, important for cation exchange between soil and roots.

It should be expected that chemical fertilizer added in solution chemically interact with soil particles before nourish through plant root. Certainly, Figure 1 shows substituted graphene model with three important functional groups for rizosphere life: The aminoacid group (at the top of model) for nitrogen root fixations processes , the carboxylic group (at the right of model) for exchange of cations ( K, Mg, etc), and the polyphosphate group (at the bottom of model) for ATP function. Certainly, these important organic groups are more probable on the carbonaceous petcoke surface rather than on the inorganic sand surface. Therefore, though petcoke may be classified as a recalcitrant organic carbon, it may also be a pool of organic carbon in the long term. Accordingly, adequateness of NPK organic compound for rizosphere life is the important parameter responsible for present results.

Closure:

The observed relative petcoke fertility respect to a sandy soil, taken as reference of desertic soil, and the above cited albedo effect on rainfall, both suggest that petcoke could be a prototype amendment for desert greening.

REFERENCES:

[1] Lehmann J, Gaunt J, Rondon M. (2006) Bio–char sequestration in terrestrial ecosystems – a review. Mitigation and Adaptation Strategies for Global Change.11: 403–27.

[2] Laine J (2012) Perspective of the preparation of agrichars using fossil hydrocarbon coke, Renewable and Sustainable Energy Reviews. 16: 5596-5602.

[3] Laine J (2013) Environmental impact assessment of the application of pyrogenic carbon in soil. J. Environmental Protection. 4: 1197-1201.

LEGEND OF FIGURES:

Figure 1: A model for substituted graphene with NPK functional groups [2]

Figure 2: XRD spectra (CuKα) in region 2Ө = 20 - 30

Figure 3: SEM of particles: Sand (top), Petcoke submitted to open sky (middle), and petcoke kept in close container (bottom).

Table I: Semi-quantitative analysis (atomic %) by EDAX.

Sample C O S Si Al Fe Mg KSand 40 43 nd 7 5 4 <1 2Petcoke1 81 17 2 <1 <1 nd nd <1Petcoke2 91 3 2 nd nd nd nd nd1: Submitted to open sky, 2: Kept in close container, nd: non-detected.

FIGURE 1

FIGURE 2

FIGURE 3 (TOP)

FIGURE 3 (MIDDLE)

FIGURE 3 (BOTTOM)

SUPPLEMENTARY MATERIAL, sunflower plant growth in 100% petcoke soil and in sandy soil:

April 02 (2014), after one week sowing 48 seeds in individual pots, approximately 50% of seeds (~12+12) sprout in each soil:

April 19, more sprouts remain on petcoke soil respect to sandy soil:

April 26, all sprouts died in sandy soil:

May 15, after transplanting into 100 % petcoke soil:

May 31, sunflowers in 100 % petcoke soil: