Activated carbon filters from agricultural waste

Olive (Olea europea L.) is an evergreen tree cultivated for the production of oil and table olives, where the world consumption of olive oil exceeds 3,300,000 tons per year (Photo 1). Olives are the leading commercial tree crop in the Mediterranean region where more than 97% of the global olive production is concentrated in the Mediterranean region.Olive oil consumption has been linked to many health-protective effects, among them slowed cell aging, and lowered age-related cognitive decline, antioxidants and antimicrobial effects. Nevertheless, the production of olive oil also results in a large amount of agricultural waste, including olive mill wastewater (OMW) and olive mill solid wastes (OMSW) which require further treatment since they contaminate the environment.
Olive oil extraction is the process of separating and extracting the oil from the olives. Today, three different extraction processes are commonly used: (i) the traditional press process, (ii) the two-phase decanter process, and (iii) the three-phase decanter process (Diagram 1). 

Diagram 1: Olive oil extraction process using the two-phase decanter process, and the three-phase decanter process

The three-phase extraction which is the main process and used mainly in Israel requires the addition of hot water to the decanter, producing olive oil, OMW, and olive cake called OMSW. Management of thelarge amount of waste, both solid and liquid, from olive oil production poses a challenge for olive mill operators and producers from both economic and environmental standpoints since so far no economical solutionsare available. The solid residues OMSW constitute a promising biomass resource because their thermochemical characteristics enable their potential utilization for energyproduction; for example,lignocellulose can be used for bioethanol production, where my research group has evaluated different processes using OMSW biomass for bioethanol production, thereby offering a solution to the management problems.

Activated carbon (AC) is considered as the most effective absorbent and commonly used for removing contaminants from gas and liquid phase including heavy metals (HMs) and organic contaminants. Nowadays, ACs are produced from agricultural wastes such as nuts shells, fruit stones, palm, coconut and other lignocellulose biomass where most of them are expensive, especially since these materials are often imported. There is a need for a cheap and locally available feedstock for the production of biochar and AC, therefore, using OMSW not only could be useful as adsorbent for the cleaning of contaminated water, but also contributes to minimizing the contamination caused by this solid waste. Biochar is charcoal produced from biomass via pyrolysis (burning without oxygen addition) used as a soil amendment as well as an adsorbent of contaminants from soil and water.

The physical and chemical properties of biochar and AC depend on the characteristics of the feedstock source and on the pyrolysis conditions, with temperature playing a key role where product yield decreases as the peak temperature increases, but higher temperatures resulting in more effective microstructure development. There are two different processes for the preparation of AC from biochar: physical activation and chemical activation.

Different projects are running in my lab at the Institute of Applied Research, The Galilee Society, to produce biochar and ACs from OMSW to be used as natural biofilter to clean contaminated water from HMs, organic industrial contaminants and nitrogen.  The presented data shows the production of biochar and AC from OMSW which was used for the removal of HMs from industrial waste water. Two different local olive cultivars (Picual and Souri) grown in Israel, two oil production processes (two-phase vs. three-phase) and two relatively low temperatures (350°C and 450°C) were used to evaluate cheaper biochar production and lower mass loss for removal of HMs from water.The study includes two parts, pyrolysis of different biomass of OMSW, and then physical activation using nitrogen gas at 900°C (Diagram 2, Fig. 1).

Diagram 2: Preparation of biochar and activated carbon from different OMSW fractions using 350°C or 450°C to produce filter used to remove HMs from water.

Figure 1: Physical activation reactor

 
In each part, the adsorption capacity was compared between different types of biochar using batch process where different HMs were dissolved in distilled water and incubated with the produced biochar or AC. Calculating the surface area was based on the Langmuir model and compared with the Brunauer-Emmett-Teller (BET) model.

The BET theory aims to explain the physical adsorption of gas/solid molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials.
Biochar production using OMSW from two olive cultivars obtained from two oil-production processes (two- and three-phase) and subjected to two temperatures (350°C and 450°C) showed 24-35% yield of the biomass, with a surface area of 1.65-8.13 m2/g, as compared to 1100 m2/g for commercial activated carbon, where physical activation of the OMSW biochar resulted in 58-70% yield and increased the surface area several folds, and the porosity of the AC ranged between 87.4-91.53(Table 1).

Table 1: The Yield (%) and the mean surface area of biochar produced at 3500C of different whole OMSW types and the porosity using BET model after physical activation. Data is mean of 3 replicates ± SD.

Picual whole OMSW from the two-phase processes, pyrolysed at 350oC, had the best removal capacity for copper (98%), lead (98%), cadmium (98%), zinc (95%), nickel (87%) and selenium (25%)after 5 minutes of incubation compared to other biochar types (Fig. 2).

 Figure2: The remaining concentration (μM) of the six heavy metals using the Picual two phases biochar obtained at 3500C or 4500C separated to Cellulose and Kernel compared to whole after incubation for 5 min. Data is mean of 3 replicates + SD.

These results suggest that surface area cannot be used as the sole predictor of HM removal capacity.The removal capacity for the HMs depended on the olive cultivar and processing type. Physical activation increased the surface area and the removal capacities of all tested HMs.
We conclude that the removal of HMs depends mainly on the cultivar type and the process used for oil extraction where Picual two-phase was better than the three-phase and similar trend was found in Souri types. All produced biochar from the different OMSW types were better than the commercial AC for HMs removal. We are currently investigating chemical as well as physical activation processes of OMSW to increase the removal capacities of HMs especially arsine which contaminate drinking water in many Asian countries.

* Institute of Applied Research,The Galilee Society, P.O. Box 437, Shefa-Amr 20200, Israel; hazaizeh@yahoo.com
Tel Hai College, Department of Environmental Sciences, Upper Galilee 12208, Israel.
Department of Natural Resources & Environmental Management, University of Haifa, Haifa, 3498838, Israel.