Exploration of cassava plant xylem for water treatment: preparation, characterization and filtration capability

Malaysia is located in Southeast Asia with high humidity, copious rainfall, and uniform temperature as a climate throughout the year (Zulhaidi et al. 2010). Its geographical location protects the country from the most major calamities such as earthquakes, volcanoes, tsunamis, and tropical cyclones (Shah et al. 2017). However, from the year 2004 to 2013, about 83% of the total number of natural disasters occurred in the Asian continent including Malaysia (Guha-Sapir et al. 2014). These hydrological disasters were classified into two categories, (i) flash flood and (ii) monsoon flood, by the Department of Irrigation and Drainage (DID) Malaysia (Ashraf et al. 2017). Based on the hydrological perspectives, flash floods take only some hours to return to the normal water level, while monsoon floods are caused by monsoon wind that can last for a month (Mohamad et al. 2012). The recent monsoon flood from December 2014 to January 2015 was regarded as one of the worst floods to hit the East Coast of Malaysia in recent decades, with more than 100,000 flood victims evacuated from their homes (Aliasa et al. 2016). This flood was a ‘tsunami-like disaster’ and called ‘Bah Kuning’ (yellow-colored flood) because of its high mud content. Due to high mud content, the water quality decreases, and the victims faced many difficulties including power loss in the affected area, destruction of the transportation system, and sustaining clean water supply for their daily activities (Baharuddin et al. 2015). Consequently, in remote areas emergency aid, especially clean water supply, came late due to road damage (Othman & Hamid 2014). Moreover, the flood caused damage to water supply facilities such as water pumps and water treatment equipment due to power loss on the affected area (See et al. 2017).

Currently, conventional and non-conventional methods were used to filter water. They all come with their advantages but might also be disadvantageous towards a certain situation. This could either be in terms of economics, environmental, or efficiency. Some of the conventional methods such as adsorption, ultrafiltration, and reverse osmosis were not suitable during a natural disaster. Reverse osmosis and ultrafiltration need a high voltage power supply to operate the water treatment system and a large place to install the system. Moreover, the chlorination method could only eliminate bacteria but the mud could not be filtered (Branz et al. 2017).

Despite the in-depth study of plant xylem, the application on plant xylem for water filtration is least explored (Boutilier et al. 2014). Therefore, most of the reported natural materials are unavailable on a large scale since natural filter materials are still being studied and analyzed. The cassava plant is an important agricultural crop which is produced annually for its roots and nutritious leaves for using in a variety of foods (Lebot 2009). Cassava is a fast-growing plant comprised of two main tissue phloem and xylem (Lebot 2009). During the transpiration process, the water travels upwards against gravitational force without applied pressure from the root inside dead xylem cells tube. The leaf has spongy and palisade surface which helps in evaporating and diffusing out water. A flow of dissolved minerals and water from roots to leaves are found in a continuous tube made from xylem cell which acts like a drinking straw (Venturas et al. 2017). This mechanism was applied to the new method discovered by Boutilier et al. which point of use (POU) filtration system was assembled from the coniferous trees' plant xylem (Boutilier et al. 2014).

Conceptually, mini pores present in the xylem tissue are possible for utilization as a filter. Previously, Boutilier et al. have reported on the performance of xylem from sapwood of coniferous tress for water disinfection. The study reported that the filter was capable of bacteria elimination and it is plausible for them to provide safe drinking water (Boutilier et al. 2014). Similarly, a recent study conducted by Ramchander et al. have reported the extensive analysis on xylem of sapwood for such application and the necessary engineering tools in designing a diverse range of xylem-based filtration products was proposed (Ramchander et al. 2021). This helpful finding divulged the idea of using natural bounty such as plant xylem to tackle the demand in developing countries during limited resources. Therefore, this study evaluated the ability of xylem tissue, specifically in cassava plants, to remove dye solution and bacteria in a water filtration system. The effects of hydrostatic pressure, length, and area diameter of xylem tissue were investigated. In addition, flow rate and filtration characteristics such as water permeability were also evaluated.

Branches were collected from cassava plants growing on private property in Kuala Langat, Selangor with the permission of the owner. The collected branches were placed in water before use. The cassava was identified as MM92 based on the number of leaves, the colour of leaves stem, and the shape of leaves (Tan & Normah 1995). Neutral red (NR) dye, analytical grade (C₁₅H₁₇N₄Cl; molecular weight 288.78 g/mol) was purchased from Friedemann Schmidt Chemical. NR dye with the range of 316.98–399.05 μm size of particle was used in this experiment. Deionized water (DI) was used throughout the experiments. An NR was prepared by dissolving 35 mg of NR dye powder in 1 L of DI water to study the NR dye solution permeability and NR dye removal of xylem tissue of the cassava plant. E. coli and total coliform count were obtained using commercial count plate from 3 M, Petrifilm, United States to analyse the presence of bacteria in Universiti Malaya's Varsity Lake water.

Xylem tissue morphology of cassava plant was determined by scanning electron microscopy (SEM) (Zeiss Supra 55 VP, Germany). The xylem tissue of the cassava plant was soaked inside the nitrogen liquid to maintain the structure of the xylem tissue. The surface and cross-sectional of the xylem structures were visualized by using SEM with an accelerating voltage of 10 kV before and after the experiment. No coating is required for visualization as it is from natural resources. Dynamic light scattering measurements of particles size distribution were performed using particle size analyser, Zetasizer (Malvern, UK) to determine the size of NR dye particles. The concentration of NR dye solution before and after filtration was measured using ultraviolet–visible (UV–vis) absorption spectroscopy (Spectrum 400, Perkin Elmer Inc., USA). It was used to measure the percentage of NR dye removal by xylem tissue from the feed solution. The dye solution was analysed using National Water Quality Standards by the Department of Environment Malaysia which are turbidity and pH. Turbidity was measured using a turbidity meter (TN50, Thermo Scientific, USA) and it was in nephelometric turbidity units (NTU). The pH was measured using a portable pH meter (HQ2100, Hach, USA).

Cassava plant or scientific name Manihot Esculenta Crantz is a tropical plant that belongs to angiosperm species of flowering plants. In the kingdom of Plantae, angiosperm species is the most diverse and largest group (Lebot 2009). It has specialized tissue and cells crucial to support, nourish, and further develop xylem and phloem that translocate nutrients and water to all plant bodies (Group et al. 2016). For some angiosperm, namely dicotyledonous trees, the process of water transportation is done by the parenchyma cells with fibrous and lignified characteristics (Rendle 1925).

Xylem parenchyma cells consist of thin-wall and tiny cells which are able to divide cells even if it is not lignified. It acts as a carbohydrate, and minerals storage with an important role in wound healing. Hence, this explains that the xylem is the most prominent tissue in trees. Additionally, much of the literature of xylem structure and function is subject to woody plants comprised of continuous tubes arranged end-to-end as the main route of water transportation (Myburg et al. 2001). The internal hydrophobic surface of the water-conducting xylem expedites the process of water transportation and offers mechanical strength to the structure (Haider et al. 2016). The wall of the xylem tissue is protected by the membrane called lignin that makes it hard so that it can resist inward pressure (Venturas et al. 2017). Due to the characteristic of the xylem tissue, the cassava plant has been examined as a medium of filter for water filtration.

Two cuts of cassava stem with a fixed length but different diameters were tested without any pressure applied or at amospheric pressure and labelled as A1 and A2 respectively. The results were recorded as in Table 1.

The stem of A1 sample with a xylem cross-sectional area of 45.23 mm² has an average water flow rate of 0.08 mL/h. On the other hand, the stem of A2 sample with a xylem cross-sectional area of 85.89 mm² has an average water flow rate of 1.25 mL/h. This shows an increase in xylem cross-sectional area will increase the water permeability across xylem tissue, significantly. The area was almost doubled yet the flow rate showed an increment of 15.6 times higher. A higher surface area will improve the filtration of the natural tissues’ membranes. This finding is in good agreement with the previously reported study which claimed that surface area is directly proportional to the conducting xylem tissue by a large factor, typically Boutilier et al. (2014).

Two cuts of cassava stem with the same length have been tested at different effective heights of the water reservoir, 17.5 cm and 88.5 cm. The set-up were labelled as P1 and P2, respectively. The effects towards flow rate were recorded as in Table 2.

The higher elevation of the water reservoir gives out a higher hydrostatic pressure head at the proximal end of the xylem. Sample P2 with higher elevation had an average water flow rate of 1.02 mL/h, while Sample P1 with lower elevation had an average water flow rate of 0.04 mL/h. In brief, the higher hydrostatic pressure of 8,681.85 Pa resulted in 25.5 times higher flow rate of 1.02 mL/h compared to the hydrostatic pressure of 1,716.75 Pa. This shows that the higher hydrostatic pressure will significantly increase the water permeability.

As the hydrostatic pressure increases, the water flux in xylem filtration also increases. Based on the formula above, where h refers to the height of the water reservoir. The greater the height of the water reservoir, the higher the pressure and water flow rate of xylem filtration. At 3240 Pa the water flow rate was 1.4 mL/h, so this means that the xylem filter could be filtered 33.6 mL per day without applied pressure from external sources, such as the mechanical pump.

Five cuts of xylem tissue of cassava plants with a fixed type of stem but different lengths were used at a similar pressure and nominated as L1–L5. The results were tabulated as in Table 3.

Sample L1 with 1.0 cm length has an average water flow rate of 0.52 mL/h and 86.24% of dye rejection. Sample L3 with a 3.0 cm length has an average water flow rate of 0.08 mL/h and 100% of dye rejection. This shows an increase in length of xylem tissue will decrease the water permeability across xylem tissue but increase the percentage of NR dye rejection. On the contrary, Ahmad Ansari et al. (2019) reported that in the case of varying length, the flow rates significantly improved as compared to varying diameter Ashoka and Silver Oak which belongs to gymnosperm group. This may be because of cassava xylem belonging to angiosperm group for which the structure and properties are different from the gymnosperm trees, as reported by Ansari et al. On the other hand, the thickness of xylem filters is a vital trade-off between flow resistance and filtration performance during E. coli removal (Ramchander et al. 2021).

The pH of the NR dye solution was 5.27 ± 0.14. However, after the filtration, the pH value becomes 7.04 ± 0.17. The turbidity of feed was 25 NTU while after filtration the value decreases to 4 NTU. Reduction of turbidity shows that the microscale geometry of the xylem tissue possesses a critical influence on its filtration performance as a natural filter medium, due to its pore size and length which are varied in different plant species (Plötze & Niemz 2011; Vitas et al. 2019). These results were comparable with the literature reported and consequently suggested that a significant reduction of turbidity enhanced the aesthetic qualities of the filtered water which is scarcely accomplished by other filtration methods such as chlorination (Boutilier et al. 2014).

The ability of the xylem filter to remove bacteria from water was investigated in this study. As a model bacterium, E. coli bacteria are cylindrical in shape with a diameter of ∼1 μm. Filtration using xylem filters in three replicates from the same stem showed nearly complete rejection of the bacteria. The bacteria can be identified by using a bacteria count plate. For this experiment, E. coli count plate was used to count the number of E. coli bacteria and total coliform in the lake water. Table 4 shows a blue colour dot that indicates the presence of E. coli bacteria while red colour dot indicates the presence of total coliform.

Table 4

Bacteria count and removal efficiency of sylem tissue

.	Before filtration .	After filtration .
		Xylem tissue 1 .	Xylem tissue 2 .	Xylem tissue 3 .
Turbidity (NTU)	21.3	3.8	3.6	4.0
Number of count	98 (red dots)	0 (red dot)	0 (red dot)	2 (red dot)
	26 (blue dots)	0 (blue dot)	0 (blue dot)	0 (blue dot)
Blue dots indicate E. coli bacteria. Red dots indicate total coliform.
E. coli bacteria removal (%)		100	100	100

.	Before filtration .	After filtration .
		Xylem tissue 1 .	Xylem tissue 2 .	Xylem tissue 3 .
Turbidity (NTU)	21.3	3.8	3.6	4.0
Number of count	98 (red dots)	0 (red dot)	0 (red dot)	2 (red dot)
	26 (blue dots)	0 (blue dot)	0 (blue dot)	0 (blue dot)
Blue dots indicate E. coli bacteria. Red dots indicate total coliform.
E. coli bacteria removal (%)		100	100	100

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From the aforementioned method, it was found that the xylem tissue achieved total rejection of E. coli. This findings is consistent with the previously reported literature which estimated at least 99.9% removal of E. coli (Boutilier et al. 2014; Vitas et al. 2019). Thus, it can be said that the xylem from cassava plant, which is the angiosperm sperm species could act as a water filter in removing specific bacterias.

Xylem tissue from the cassava plant has a porous material membrane that acts as a filter in the water filtration system. The membrane in xylem tissue has an effective area for filtration by providing a moderate flow rate without applied pressure. Xylem tissue was prepared by removing the bark and vascular cadmium of the cassava stem and inserting the xylem tissue inside a tube. Removal of NR dye experiment revealed a reduction of turbidity water and ability of xylem to filter big size particles. The xylem filter could effectively filter dye particles and bacteria from water with rejection exceeding 99.9%. The flow rate of 1.25 mL/h was obtained through 1.5 cm² filter areas without applied pressure due to the soft xylem tissue structure which possibly rupture the xylem structure if any pressure was applied. The application of agricultural by-products as filter bed material for the treatment of water is preferable due to their biodegradability and availability. Throughout the experiment, the xylem tissue of the cassava plants showed a promising result in water filtration and the quality of the filtrate showed that safe drinking water can be obtained. Further research and development of xylem filters could potentially lead to their widespread use and greatly reduce the water contaminants in the world.

The authors would like to acknowledge research funding provided by the Ministry of Higher Education Malaysia through the Fundamental Research Grant Scheme (FRGS/1/2015/TK05/UM/02/1) and the Universiti Malaya Graduate Research Assistant Scheme (RP034D-15AET).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.