Scaling-Up of the Production of Biochar from Olive Tree Pruning for Agricultural Use: Evaluation of Biochar Characteristics and Phytotoxicity

Overview

Crespo-Barreiro and colleagues scaled olive-tree-pruning biochar production from semi-pilot to pilot and industrial pyrolysers, then measured heavy metals, PAHs, pH, carbon stability, and germination-index phytotoxicity. The source is primary evidence for agricultural-input contamination and soil-amendment quality control, not food occurrence evidence: it measures biochar made from olive pruning residues and does not report metal concentrations in olives, olive oil, soil after application, or edible crops.

Key numbers

Biochar production codes

Code	Reactor scale	Temperature	Residence time
B400	Semi-pilot	400 +/- 3 C	40 min
B500	Semi-pilot	500 +/- 3 C	40 min
B600	Semi-pilot	600 +/- 3 C	40 min
BF600	Pilot	600 +/- 15 C	40 min
BMEC	Industrial	666 +/- 50 C	40 min

The industrial run processed 2330 kg of material. The pilot run processed 70 kg. The semi-pilot batches used 200 g feedstock.

Heavy metals in biochar

Table 4 reports total concentrations in mg/kg, mean +/- SD, n = 3. Censored values are shown as the source reports them.

Metal	B400	B500	B600	BF600	BMEC	IBI 2015 threshold	EBC-Agro 2022 threshold
arsenic (As)	<4	<4	<4	<4	<4	13-100	13
cadmium (Cd)	<0.8	<0.8	<0.8	<0.8	<0.8	1.4-39	1.5
Co	0.49 +/- 0.001	0.86 +/- 0.02	0.99 +/- 0.04	0.89 +/- 0.04	2.76 +/- 0.07	34-100	not listed
chromium (Cr)	1.84 +/- 0.01	10.33 +/- 0.02	40.99 +/- 0.08	21.44 +/- 0.07	129.34 +/- 0.13	93-1200	90
Cu	115.41 +/- 0.19	137.52 +/- 0.26	102.42 +/- 0.32	149.26 +/- 0.09	221.46 +/- 0.33	143-600	100
mercury (Hg)	<1	<1	<1	<1	<1	1-17	1
Mo	<2	<2	<2	<2	2.33 +/- 0.01	5-75	not listed
nickel (Ni)	<2	5.8 +/- 0.08	19.07 +/- 0.08	10.75 +/- 0.13	75.64 +/- 0.29	47-420	50
lead (Pb)	<4	<4	<4	<4	5.99 +/- 0.32	121-300	120
Se	<4	<4	<4	<4	<4	2-200	not listed
Zn	43.26 +/- 0.31	56.91 +/- 0.21	36.69 +/- 0.22	49.98 +/- 0.33	51.31 +/- 51.31	416-7400	400

All samples were below IBI 2015 thresholds. The authors state that EBC-Agro 2022 is stricter: Cu exceeded the EBC-Agro limit in all five samples, while the industrial BMEC sample also exceeded the EBC-Agro chromium (Cr) and nickel (Ni) limits. The authors attribute Cu plausibly to copper phytosanitary treatment of olive trees followed by concentration during pyrolysis. They report the overall heavy-metal burden decreasing in the order BMEC > BF600 > B600 > B500 > B400.

PAHs, pH, and phytotoxicity

Total EPA 16-PAHs were 6.1 mg/kg (B400), 6.4 mg/kg (B500), 5.8 mg/kg (B600), 34.9 mg/kg (BF600), and 2.6 mg/kg (BMEC). Naphthalene was the dominant PAH in all samples and reached 28 mg/kg in BF600. The authors state that all samples met IBI PAH specifications, while B600 and BMEC were below the EBC-Agro 6 mg/kg limit, B400 and B500 were just above it, and BF600 exceeded it.

Biochar pH rose with scale/temperature: B400 8.14 +/- 0.18, B500 8.70 +/- 0.24, B600 9.10 +/- 0.06, BF600 9.71 +/- 0.19, BMEC 11.52 +/- 0.43. Electrical conductivity stayed below 1.80 dS/m.

Germination index (GI, %) showed that B600 was phytostimulant for all four test crops, while BMEC was phytotoxic for all four:

Code	Tomato GI	Cress GI	Radish GI	Lettuce GI
B400	5.2 +/- 0.018	119.9 +/- 2.69	79.9 +/- 0.26	4.6 +/- 0.23
B500	108.4 +/- 1.76	56.6 +/- 0.69	275.3 +/- 1.56	19.1 +/- 0.16
B600	130.5 +/- 2.54	249.7 +/- 2.80	368.3 +/- 2.29	162.3 +/- 1.97
BF600	63.0 +/- 1.32	2.4 +/- 0.19	20.8 +/- 0.30	53.5 +/- 1.89
BMEC	8.3 +/- 0.36	1.2 +/- 0.06	19.1 +/- 0.62	3.2 +/- 0.09

The authors interpret BMEC phytotoxicity primarily as an alkalinity problem (pH 11.52), while for samples with pH closer to neutrality they saw a tighter relationship between PAH concentration and phytotoxicity.

Carbon stability

R50 and gained-stability indices increased with production temperature and scale. Reported non-labile carbon stability was 90.00% (B400), 91.53% (B500), 92.50% (B600), 92.12% (BF600), and 94.05% (BMEC).

Methods (brief)

Olive pruning residues from a Spanish olive producer were pyrolysed in three reactor systems. The semi-pilot reactor tested 400, 500, and 600 C under helium carrier gas; the 600 C condition was then scaled to pilot and industrial equipment. Heavy metals were extracted from 0.5 g biochar with HCl/HNO3 and measured by ICP-OES, following European Biochar Certificate and International Biochar Initiative framing. PAHs were measured by Eurofins Umwelt Ost GmbH using hot toluene extraction and GC-MS according to DIN EN 16181:2019-08. Phytotoxicity used Zucconi-style germination index assays in tomato, cress, radish, and lettuce with five replicates per treatment. Carbon stability used the R50 thermogravimetric index after washing.

Implications

Certification: This source is useful for supplier-questionnaire and agricultural-input controls: olive-derived biochar can carry enough Cu, Cr, and Ni to exceed strict biochar/agricultural-amendment limits, particularly when industrial-scale temperature control drifts high. It should not be interpreted as evidence about olives or olive oil metal occurrence.

Courses: Good case study for the distinction between circular-economy agricultural inputs and food-matrix evidence. The same olive supply chain can produce a useful soil amendment, but the amendment’s contaminant profile still needs analytical verification before field use.

App: Route as agricultural-input context for biochar and soil amendments, especially where biochar is proposed as a heavy-metal immobilization or soil-health intervention.

Update history

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