Assessment of physicochemical parameters and trace metal elements from untreated and treated wastewater of an analysis laboratory, Yaoundé-Cameroon

ABSTRACT The main emphasis of this study is to assess the status of daily released effluent before and after treatment with both Corn Cobs Activated Carbon (CCAC) and eggshells, with a focus on raising its pH to acceptable levels for agricultural irrigation. The physical properties of CCAC revealed that, it contained 5.82 % ash and had a point zero charge of 5.35. Before and after each treatment, the Electrical Conductivity and Total Dissolved Solids ranged from (248.3 ± 0.9) µS/cm to (285.3 ± 0.7) µS/cm and (124.00 ± 0.56) mg/L to (142.33 ± 0.34) mg/L, respectively; the pH of 6.98 ± 0.03, was found to be within the Food and Agricultural Organization of the United Nations (FAO) allowable limits for irrigation purposes. The average mean concentrations analysis of trace metal elements with Inductively Coupled plasma Optical Emission Spectrometry ranged from (0.000 ± 0.000) mg/L to (0.76 ± 0.02) mg/L for untreated and treated wastewater. After the final treatment, the concentrations of Fe2+ ions were lower than those of Zn2+, Cu2+, and Mn2+, but they were all within the FAO’s permissible limits. Similarly, the mean values for the removal efficiency of trace metal elements revealed that, Zn2+, Cu2+, and Mn2+ ions are 100 % decontaminated, whereas only 84.33 % Fe2+ ions are removed with 500 grams of eggshells. There are both positive and negative correlations between physicochemical parameters and trace metal elements. The Principal Component Analysis results of two latent factors explain 61.95 % and 29.14 % of the total variance into three groups, with all trace metal elements forming a single group. The one-way ANOVA revealed a highly significant difference between parameters and treatments at p 0.05. These findings suggest that, CCAC and eggshells have shown promising applications for industrial wastewater treatment and can be easily adopted by analytical laboratories.


Introduction
Trace metal elements have been excessively released into the environment due to rapid industrialisation and have created a major global concern.Cadmium, zinc, copper, nickel, lead, mercury and chromium are often detected in industrial wastewater, which originates from metal plating, mining activities, smelting, battery manufactures, tanneries, petroleum refining, paint manufactures, pesticides, pigment manufactures, printing and photographic industries [1].Many of these activities generate liquid effluents that may contain many different chemicals, as well as organic matter, depending on the nature of the industrial processes involved.Due to their biomagnification, non-biodegradability and toxicity even at low concentrations, trace metal elements are very harmful, both for humans and for the whole environment [2,3].It was observed that in Cameroon, many industries who carry on chemical analyses do not control the pH of their effluents before they are discharged into the environment.Infiltration of such untreated acid wastewater loaded with high concentrated trace metal elements lead to acidification of soil and subsequently of stream water [4].Soils polluted by trace metal elements have become a major concern in the world, not only because of their toxicity and their persistence in the ecosystem but also due to their degradation [5].In addition, trace metal elements in agricultural soil can be incorporated into the food chain through agricultural practices such as food crops irrigated with wastewater [6,7].Irrigation with wastewater without proper treatment can lead to plant uptake of trace metal elements, causing serious health problems for humans and animals as well as the environment [8][9][10].Assessment and monitoring programmes for waterbodies receiving industrial waste should include variables selected to indicate background water quality in relation to the local industrial processes, especially contaminants that may cause harm to the environment or make it unsuitable for other uses.For this reason, it is important to develop not only effective and reliable treatments for the removal of these ions but also to control the acidity of wastewater before it is discharged as specified by the UN' SDG 6. Wastewater management encompasses a broad range of efforts for a sustainable discharge.Thus, it is becoming a tedious process for many industries with regard to environmental norms and legislations.
The role of the wastewater treatment plant operator has become very important in the prevention of environmental degradation in industries.A number of technologies for the removal of trace metal elements have been developed over the years, they include chemical precipitation, filtration, evaporation, ion-exchange, reverse osmosis, oxidation and filtration.Though efficient, they also pose some disadvantages via the production of toxic sludge, high-energy requirements, time consuming and the inability to regenerate the starting materials [11].Among these methods, the adsorption process implies the presence of a solid adsorbent that binds molecules/ions by physical attractive forces, ion exchange and chemical binding.The adsorption technique is a non-destructive method in which the pollutants are transferred from one phase to another.It has shown good processability with various adsorbents such as activated carbons, clays, zeolites, polymeric, etc. for the removal of trace metal elements from wastewater [12,13].Although many sorbents including chitosan, sunflower stalk, shale oil ash, orange peel, natural clay, soy meal hull, citrus-peels, sawdust, and eggshells have been undertaken on wastewater treatment plants [14,15], less attention has been paid to the corn cobs by-product.
During the processing and production of corn, several wastes are generated including corn cobs and corn husks [16].Maize, the raw material chosen for this study, is widely grown in Cameroon and in the Central African sub-region.In most cases, the residues from the primary processing of maize, which consists of foliage, stalks and cobs, except in the arid Sahel North of the country where they are used as low-grade fuels for domestic heating or for cattle nutrition, are usually disposed in the open air and not fully utilised, which cause both environmental pollution and wastage of natural resources [17,18].Therefore, reasonable and harmless utilisation of corncobs is important to change agro-waste into valuables.Due to their relatively high abundance, availability and being not saleable, corncobs are good candidates for producing activated carbon in order to enhance the development of sustainable adsorbents thereby contributing to the recovery of agricultural wastes [19,20].Activated carbon is one of the most popular adsorbent materials, which is widely used in wastewater treatment applications.Commonly, most of the activated carbons are produced by physical activation, chemical activation and physical-chemical activation [21].Physical activation involves carbonisation of carbonaceous materials followed by activation of the resulting char in the presence of activating agents such as steam at relatively high temperature [22,23].Chemical activation is known as a single step method for producing activated carbon in the presence of chemical agents such as inter alia zinc chloride, potassium hydroxide, sodium carbonate and phosphoric acid [24,25].Chemical activation is done by impregnating the material with an activator and then performing a heat treatment between 400 °C and 900 °C, where the process of preparing activated carbon is carbonised and activated simultaneously [26].Among activating agents, phosphoric acid is one of the most commonly used for environmental and production costs because it plays a vital role in the chemical activation process including the degradation of biomass and the formation of oxygen-containing surface functional groups [17].
Chicken eggshells are waste materials from hatcheries, homes and fast-food industries which are abundant and readily available worldwide [27].Without proper management, these solid wastes contribute significantly to environmental pollution by their smell, they provide the perfect habitat for flues, damage the nearby environment and also cause some allergies to people working around areas when they are kept for long time.A recent report described that chicken eggshells contain various organic molecules and mineralised components such as calcite which are combined in several layers [28].The presence of a cellulosic structure and amino acids allow metal ions and large organic nutrients to be cross-linked with and therefore make it a good bio sorbent [29].
Keeping in view that CornCobs Activated Carbon (CCAC) has not yet been used to reduce trace metal elements concentration and that the enhancement of wastewater's pH with eggshells prior to discharge is still a challenge to overcome, the aim of this study is therefore to ascertain the current status of physicochemical parameters and trace metal elements present in daily raw effluent released after routine analyses and to compare their values with those of water quality for agricultural irrigation.

Collection and of corn cobs and eggshells
The corn cobs were collected from the antenna of IRAD Ntui, found in the humid agroecological forest zone of Cameroon.Chicken eggshells were collected from Food Technology Lab at IRAD Nkolbisson.The samples of chicken eggshells were washed with distilled water several times to remove dirt and contaminants, followed by drying in a hot air oven at 110 °C for at least 12 hrs.Thereafter, the chicken eggshells were then removed and ground using pestle and mortar.The sample was stored in an airtight container for future use.

Pretreatment of corncob
The raw material was washed several times with tap water to remove dust and subsequently thoroughly cleaned with distilled water to get rid of impurities before drying in an oven for 72 hrs at a temperature of 105 °C [18].After drying, the raw material was crushed with a local pestle and sieve (sieve type; AFNOR X11-501) to different particle sizes of 2 mm (Fine Particles), ≤ 3.15 mm (Medium Particles), and > 3.15 mm (Coarse Particles).It was then dried in an oven at 105 °C to a constant weight and placed in a desiccator for use.

Impregnation of raw material
Corn cobs were soaked in phosphoric acid (H 3 PO 4 ) with an impregnation ratio of 1:5 (w/w) for 24 hrs.The samples were stirred every 3 hrs to ensure that the activator going into the raw material.The impregnated samples were placed in the oven to dry at 105 °C until a constant weight.

Carbonisation and activation
The carbonisation was done at 400 °C and 600 °C for 1 hr in a muffle furnace and at the end of the process activated carbon was obtained.After cooling the resultant Activated Carbon (AC) was collected, its pH was measured and adjusted by washing with distilled water and NaOH to obtain a neutral value.Thereafter, the samples were dried in an oven set at 110 °C for 24 hrs, and stored in a desiccator containing silica gel for further experiments.

Determination of ash content
The standard test method for ash content-ASTM D2866-94 [30] was used.A crucible was pre-heated in the furnace to about 650 °C for 140 minutes, cooled in a desiccator and weighed.30 g of activated carbon samples were transferred into the open crucibles and heated at (650 ± 5) °C for 2 hrs in a furnace.They were removed and allowed to cool in a desiccator to room temperature and reweighed.The weights of the sample before and after heating were used to determine the amount of ash in the sample.

Determination of the point zero charge (pHzpc)
The point zero charge (pHzpc) of the activated carbon was determined following the procedure of Berges et al. (2021) with little modification [31].More precisely, 0.01 M of NaCl solution with different pH values in the range of 2-10 was prepared by adding HCl/NaOH.Thereafter, 0.20 g of activated carbon was added to 50 mL of 0.01 M NaCl solutions of different pHs in reagent bottles.The samples were agitated after allowing them to react for 48 h at room temperature and the pH of each solution was measured.

Process design and implementation
• Three sets of transparent containers with their respective seals were required to mount the cartridge adsorption unit, of which the first set made up of three containers of 9 L in volume was to serve as collection tanks, the second made up of three containers of 13 L in volume to serve as cartridge carriers and the last set made up of 3 containers of 1 L in volume to serve as cartridges.
• The length and diameter of each set of containers were measured to be 26 cm by 28 cm for the collection tanks, 24 cm by 21 cm for the cartridge carriers and 17.5 cm by 9 cm for the cartridges.• Carved openings of 9 cm in diameter were made at the centre of the bottom of each cartridge carrier container.The three cartridges were respectively inserted into the various carved openings and sealed with the help of a PVC gum for them to stick to the bottom of the cartridge carriers.
• An opening was provided on each cartridge seal to ease the insertion of the adsorption material and very tiny openings provided at the bottom of the various cartridges to permit the flow of water.• The seals of the collection tanks were carved to a diameter of 9.5 cm each and the various cartridge carriers plugged-in making an overall of 3 distinct setup units.The setup units were organised and arranged in a top to bottom manner to permit gravitational flow.• Taps were provided on the collection tanks of the different setup units to ease the retrieval of water and their interconnection with the help of transparent extension pipes.Once the entire design was set up, the different cartridges were filled with the prepared adsorbents.

Cartridge preparation
The length of each cartridge was measured to be 17.5 cm and filled respectively with activated carbon and cotton of different particle sizes at equal length intervals of 4 cm each ranging from fine particles AC (FP), medium particles (MP) and coarse particles AC (CP) arranged progressively from bottom to top of the cartridge.Cotton was used as a separating layer for each filter media and at the bottom perforated end of the cartridge to prevent the filter media from leaking out during the filtration process (Figure S1).

Quality control and assurance
The analyses were carried out at the Laboratory of Soils, Plants, Water and Fertiliser Analyses (LASPEE) of the Institute of Agricultural Research for Development (IRAD), Yaoundé.All the equipment used were calibrated with standard reference materials such as standard solutions of 4.01, 7.01 and 10.01 for pH-meter, KCl 0.01 mol/L for the conductivity-meter and multielement solutions of As, Be, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Mo, Ni, Pb, Se, Sr, Ti, Tl, V, Zn for ICP-OES before any readings.This was to ensure their reliability and the accuracy of the readings undertaken on the instruments.The chemicals used were of analytical reagents grade with high purity.All solutions were prepared with distilled water.

Sample collection and characterisation
About 5000 mL of wastewater sample was collected directly from the storage container of the ICP-OES apparatus.The wastewater of the storage container used for this study was derived from analyses done both on stream and groundwater as well as other laboratory routine analyses.Physicochemical parameters like pH were determined using a pH-meter (HANNA Edge) on the raw and each treated effluent.The Electrical Conductivity (EC), Temperature (Temp.),Salinity, Total Dissolved Solid (TDS) were instantly measured and recorded for the various steps any step using a conductivity meter (InoLab Cond 1).The Turbidity, Suspended Matter (SM) and Colour were assessed with a photometer (Wagtech 7500) before and after each treatment.The trace metal elements viz Fe 2+ , Mn 2+ , Ca 2+ , Cu 2+ and Zn 2+ were assessed in the raw effluent and after the three treatments (Figure S1) through Inductively Coupled plasma Optical Emission Spectrometry (ICP-OES Optima 8000, Perkim Elmer) instruments.The Optima 8000 ICP-OES is a dual view instrument, allowing for both axial and radial plasma readings, which enables the measurement of high and low concentration samples during analysis .The use of compressed air as a shear gas removes the plasma tail plume, which eliminates many interferences and minimises the need for the addition of ionisation suppressants.The photons generated within the plasma are measured by a highly sensitive photon detector, Charge-Coupled Device.The ICP-OES sample experiences temperatures estimated to be in the vicinity of 10,000 K.This results in atomisation and excitation of even the most refractory elements with high efficiency.ICP-OES has advantages in terms of detection and quantification limits as well as speed of analysis.

Processing method
Prior to processing, distilled water was passed thrice through the cartridges to determine the flow rate of each.The flow rate was then set to 8 mL/sec for the cartridges loaded with Corn Cob Activated Carbon (CCAG) and 3.8 mL/sec for the cartridge loaded with eggshells.The raw water residue (WW0) from the Inductive Coupled plasma Optical Emission Spectrometry (ICP-OES Optima 8000, Perkim Elmer) instruments was passed through the first activated cartridge packed with 200 g activated carbon.After flowing, 500 mL of the sample WW1 was collected in a polyethylene bottle pre-washed with laboratory detergent and rinsed with distilled water.The physicochemical parameters were immediately determined and trace metal elements including Fe 2+ , Mn 2+ , Ca 2+ , Cu 2+ and Zn 2+ were determined at the wavelengths of 259.940, 294.020, 317.933, 322.393 and 205.200 nm, respectively, in mg/L using ICP-OES Optima 8000, Perkim Elmer.The ICP-OES was calibrated with relevant grade standards.Sample WW1 from the first treatment was then passed through the tap connected to the second cartridge containing 200 g activated carbon for a second treatment.After adsorption, 500 mL of the sample (WW2) were collected and analysed for physicochemical parameters and trace metal elements.Finally, the sample WW2 was transferred to the third cartridge loaded with 500 g eggshells and allowed to flow completely.500 mL of the sample WW3 was collected and the parameters analysed.

Treatment efficiency
Four samples of 500 mL each were collected in polyethylene bottles which were subsequently pre-washed with laboratory detergent and rinsed with deionised water.The removal efficiency (%) of Fe 2+ , Mn 2+ , Cu 2+ and Zn 2+ were determined for each metal ion flowing from each cartridge using equation 1.
Where: R: the removal from a single metal concentration (i) passing through the cartridge.C i : initial concentration of each metal in mg/L from the residual effluent WW0.C f : Remaining concentration of each metal after the treatments WW1, WW2 and WW3.

Statistical analyses
The entire data set was computed for correlations matrix using XLSTAT 2020 (trial version).In order to evaluate the relationships between samples and parameters, the Principal Component Analysis and Pearson's correlation matrix were executed for 13 parameters of the 4 samples including one sample from the residual raw effluent and three resulting from the different treatments.For all comparisons between four experimental groups, paired Student's tests were performed.For multiple group comparisons, One-way analysis of variance (ANOVA) and Tukey multiple comparisons were applied using Jmp Pro 16.1 version 2021 for evaluating the significant differences between the raw effluent and the treated effluent.

Characteristics of the activated carbon
The point of zero charge is the pH value at which a surface presents a net charge equal to zero.A negative net charge is caused on the surface by pH values superior to pHpzc, while inferior values cause a positive charge [31].The point zero charge (pHzpc) of AC was found to be 5.35 indicating that the AC is slightly acidic.The surface of AC becomes positively charged at pH < pHzpc favouring the adsorption of anionic pollutants, whereas cationic pollutant sorption would be favourable at pH > pHzpc.In our study, the pH for the treatments were pH 5.86, 5.88, 5.82 and 6.98, respectively, for WW0, WW1, WW2 and WW3 confirming a good adsorption of trace metal ions.The ash content of corn cob activated carbon was 5.42 % suggesting a low mineral composition.

Physicochemical parameters
The mean values of physicochemical parameters such as pH, temperature, EC, salinity, TDS, Turbidity (FTU) and Colour Pt-Co are presented in Table 1.

Temperature
Temperature, the measure of intensity of heat stored in a volume of water, is highly correlated with atmospheric temperature and the morphometric features.As observed in Table 1, the average water temperature from the four treatments ranges from (22.4 ± 0.2) °C for the raw wastewater WW0 to (20.27 ± 0.09) °C for the wastewater treated with eggshell (WW3).The variation may be dependent on geographical location and meteorological conditions such as humidity and incoming solar radiation in the experiment hall.

pH
pH is a valuable parameter that guides not only the status of acid-alkali balance of the water but also serves as an important index of the degree of pollution.The average mean values of pH for the raw effluent and the three treatments show weakly acidic pH values of 5.86 ± 0.01, 5.88 ± 0.01, 5.82 ± 0.02 and 6.98 ± 0.03, respectively, for WW0, WW1, WW2 and WW3 (Table 1).However, the values of WW0, WW1 and WW2 were closed to those obtained by Moussima et al. (2020) [32] during the assessment of physicochemical and bacteriological quality of groundwater and health risks in some districts of Yaoundé VII, Cameroon.These values are out of the pH range for irrigation water (6.5 to 8.4) indicating that, the water quality was abnormal when treated twice with CCAC [33].In addition, irrigation with such water could affect metallic plumbing fittings and may cause a nutritional imbalance affecting plant growth and health [34].After the last treatment with eggshells (Figure 1), the value of pH was boosted to 6.98 ± 0.03 for sample WW3 in such a way that it falls within the safe limit of 6.5 to 8.5 set by FAO and can therefore be used for agricultural irrigation [35].This slight increase of pH may be due to the release of calcium carbonates (CaCO 3 ) from eggshells into solution [36].

Electrical conductivity (EC) total dissolved solids (TDS), and suspended matter (SM)
The EC measures the capacity of a solution to carry an electrical current and is a total parameter for dissolved and dissociated substances which also indicates the concentration of dissolved electrolytes.The average mean values for electrical conductivity in Table 1 follows the trend (268.67 ± 7.50) µS/cm, (285.33 ± 1.15) µS/cm, (279.33 ± 0.57) µS/cm and (247.33 ± 1.53) µS/cm, respectively, for WW0, WW1, WW2 and WW3.The slight increase in electrical conductivity when treated with CCAC could be due to a proton switch mechanism among the activation agent H 3 PO 4 and the carbonaceous structure of the corn cob [37].All the values were very low when compared with the standard limit of water quality for agricultural irrigation indicating a non-saline water [34].Thus, using this water directly for irrigation may not affect plant growth.
The TDS is an important parameter for distinguishing among mineral and saline waters.In the present study, the average mean values given in Table 1 are (134.67± 5.13) mg/L, (142.33 ± 0.34) mg/L, (139.67 ± 0.34) mg/L and (124 ± 0.56) mg/ L, respectively, for WW0, WW1, WW2 and WW3.These values are in the range of 50-500 mg/L required by FAO [35] suggesting a low mineral concentration in the samples treated [38].
The determination of suspended matter (SM) is a very important parameter to assess the quality of effluent discharged and the appropriate treatment method [39].Table 1 shows a high level of SM in mg/L for WW0 (68.34 ± 23.20) and WW3 (78.77 ± 4.26) and low values for WW1 (11.12 ± 2.78) and WW2 (23.17 ± 6.10).This can be explained due to the fact that the treatment with CCAC has the potential of retaining suspended matter in the pores of the CCAC resulting in their decrease.The high value of SM observed in the last treatment may be attributed to the presence of the insoluble mucin protein of the outer surface of eggshells [36].

Turbidity, salinity and colour
The turbidity is the measure of relative clarity of a liquid and refers to the content of a fluid material that makes it troublesome.It measures the extent to which light is either absorbed or scattered by suspended material in water.The mean values given in Table 1 for WW0 (49.17 ± 16.69), WW1 (8 ± 2), WW2 (16.68 ± 2.91) and WW3 (56.67 ± 1.74) are a characteristic of grey wastewater and non-saline water resulting from the cleaning of ICP-OES during analyses [39,40].
The colour value was high for the initial effluent WW0, but becomes progressively low after the WW 1 and WW2 treatments because of activated carbon which has a clarifying power, hence the decrease in colour intensity.In the third treatment WW3, the colour increases again.This could be due to the outer residual protein of the eggshell's surface [40].Additionally, the gelation of egg proteins usually occurs in a two-step process: the first step involves changes in the conformation or partial denaturation of protein molecules; in the second step, further aggregations of denatured proteins leading to an exponential increase of the colour [41].

Calcium
Calcium (Ca 2+ ) ions are naturally present in water and may dissolve from rocks such as limestone, marble, calcite, dolomite, gypsum, fluorite and apatite.In plants, intracellular changes in free Ca 2+ levels act as regulators in many growth and developmental processes by mediating the cellular responses to environmental stimuli and thus play an important role in providing stress tolerance to plants [42].The concentrations of Ca 2+ vary from (12.67 ± 0.67) mg/L for WW0 to (18 ± 2) mg/L for WW3 (Table 1).The decrease in Ca 2+ ions observed during the treatments WW1 (10.67 ± 0.34) mg/L and WW2 (9.89 ± 1.34) mg/ L show a slight fixation of Ca 2+ ions in the pore of CCAC.The increase in the treatment with eggshells could be due to the addition of remaining Ca 2+ ions present after the treatments with CCAC and those released from the dissociation of calcium carbonate of the eggshells.According to FAO, Ca 2+ content did not exceed the permissible level (0 -20 mg/L) recommended for irrigation water [35].Thus, the raw and treated waters exhibit no Ca potential hazards.

Copper
Even though Copper (Cu) is an essential element for humans and plants when present in smaller amounts, in excessive amount it exerts detrimental effects on crops.The concentrations of Cu 2+ ions were found in only three samples, i.e.WW0, WW1 and WW2 with values (0.15 ± 0.02), (0.10 ± 0.02) and (0.033 ± 0.0207) mg/L, respectively, as shown in Table 1.The concentration of copper in the treatments WW2 and WW3 is less than the permissible limit of FAO (0.20 mg/L) [35], the possible cause of their decrease during the treatment could be related to complexation occurring through the electron pair sharing between electron donor atoms (N and O) present in the functional groups of activated carbon and the Cu ions [43].Additionally, the absence of copper in the sample WW3 (0.000 ± 0.000) mg/L signifies a total retention of the ions on the eggshells.This indicates the ability of eggshells waste which is mainly constituted of calcium carbonate to bind all the Cu 2+ ions through electrostatic interaction between the metal ions and the carbonate group [44].

Iron
Iron (Fe) is a naturally occurring metal in the form of magnetite, haematite, etc., and it gets into water during the extraction of the metal from its ore.It also enters into the water when aluminium waste products containing iron are discharged into water.As shown in Table 1, the concentrations of Fe 2+ decrease from (4.75 ± 0.11) mg/L for the untreated water WW0 to (0.76 ± 0.02) mg/L when treated with eggshell.During the first treatment, the concentration of Fe 2+ decreases up to (1.73 ± 0.18) mg/L because of Fe ions fixation on the active site of activated carbon.Similarly, the second treatment also induced a decrease to (0.78 ± 0.05) mg/L resulting as result of another retention of the unfixed Fe ions after the first treatment with activated carbon.The concentration of (0.76 ± 0.02) mg/L obtained after the treatment with eggshells may be attributed to electrostatic repulsion and valence forces through exchanging and sharing of electrons between the atom donor on eggshell proteins and the Fe 2+ ions receptor [43].The quantities of Fe 2+ ions in all the samples are below the permissible limits allowed in irrigation water as suggested by FAO (5 mg/L) [35], their presence in wastewater used for irrigation causes accumulation in soils where there is a potential that they could become bioavailable for crops and reduce protein synthesis in leaves [45].

Manganese
Manganese (Mn) is an essential trace element abundantly present in earth crust in the form of oxides and hydroxides.Manganese commonly finds its way into water through activities such as industrial emissions, soil erosion, volcanic emissions and human activities such as burning.Excessive Mn 2+ concentration in plant tissues may cause alteration in various processes, including enzymatic activity, nutrient uptake behaviour and redistribution, thereby antagonistic effects on the use of other major nutrients (Ca, Fe, Mg, N) [46].The concentrations of Mn 2+ (mg/L) followed the trends WW0 (0.004 ± 0.000) > WW1 (0.0017 ± 0.0004) = WW2 (0.0017 ± 0.0004) > WW3 (0.000 ± 0.000) as given in Table 1 and Figure 2.These results indicate that there is a low Mn 2+ ions fixation on treatments with CCAC due to its ionic radius that is larger than that of Zn 2+ , Cu 2+ and Fe 2+ ions and does not allow it to compete in solution with other trace metal ions.The concentration of (0.000 ± 0.000) mg/L obtained after the treatment with eggshells may be attributed to total binding through electrostatic interaction between the Mn 2+ ions and the carbonate group of eggshells [43].All the values do not exceed the permissible limit of FAO (0.20 mg/L) [35] and therefore are not harmful to the soil environment and consequently not detrimental for crops because high concentrations of Mn 2+ ions could impede chlorophyll biosynthesis, photosynthetic rate, carbon dioxide (CO 2 ) assimilation rate and stomatal conductance and consequently affect spinach growth and development [47].

Zinc
Zinc (Zn) is an essential micronutrient with a series of critical roles in living organisms; yet, Zn persistence in nature can be toxic at elevated concentrations and affects plant growth and development, as well as threatens the health of animals due to excessive amounts in the food chain [48].From Table 1, the concentration of Zn 2+ decreases from (-0.73 ± 0.02) mg/L to (0.00 ± 0.00) mg/L and inversely the pH increases from 5.86 ± 0.01 to 6.98 ± 0.03 thus, reducing the toxicity of this element at the last treatment with eggshell.The high concentration of Zn 2+ in the raw effluent WW0 may be due to the dissolution of Zn 2+ from water pipelines whereas, it decreases in the treatment WW3 is due to electrostatic repulsion and valence forces through the exchange and sharing of electrons between the atom donor of eggshell proteins and the Zn 2+ ions receptors [43].
Even though the concentrations of Zn 2+ are within limits set by FAO (2 mg/L) [35], bioaccumulation of these ions in irrigation without proper treatment will be toxic not only for plants but could acidify both soils and surface waters [49].

Treatment efficiency of trace metal elements of corn cob activated carbon and eggshell
The percentage of trace metal elements adsorbed (Table 2) was calculated using equation 1.0.Mn 2+ , Cu 2+ and Zn 2+ were completely decontaminated when 200 g of Corn Cobs activated carbon and 500 g of eggshells were used successively as shown in Figure 2. The percentage removal of Zn 2+ was significantly different for WW1 (84.41 ± 1.42) %, WW2 (93.08 ± 1.60) % and WW3 (100.0 ± 0.00) %.From Figure 2, the percentage of Zn 2+ ions adsorbed was very high compared to Fe 2+ , Mn 2+ and Cu 2+ between the same treatments showing an easy fixation of Zn 2+ ions in relation to other ions.The reason may be due to the small ionic radius of Zn 2+ ions which facilitates its adsorption on porous materials like activated carbon and eggshell.The difference between the decontamination percentages of Fe 2+ revealed that the treatment WW1 (63.49± 7.41) % is significant difference with the treatments WW2 (83.58 ± 1.17) % and WW3 (84.33 ± 0.74) % but not significantly different between WW2 and WW3.The removal efficiency of Fe 2+ ions in WW2 and WW3 being equal could suggest a very little retention of these ions on eggshells, the percentage decontamination is less than 100 % and the percentage adsorbed is high compared to 84.32 ± 1.17 a 100.00 ± 0.00 a 100.00 ± 0.00 a 100.00 ± 0.00 a Mn 2+ and Cu 2+ in the treatment WW2.In addition, even though 100% decontamination was achieved with both Mn 2+ and Cu 2+ in the last treatment, their removal percentage is low during their retention of activated carbon.Similar removal percentages have also been observed for Cu 2+ and Zn 2+ by Sabry et al. (2015) in their on Metal Ion Removal from Wastewaters by Sorption on Activated Carbon, Cement Kiln Dust, and Sawdust [50].Moreover, 100 % uptake of Mn 2+ , Cu 2+ and Zn 2+ by eggshells at a pH of 6.98 ± 0.03 may be attributed to calcium contents, which raise the pH at the last treatment [51].

Correlation among the parameters studied
A good relationship between two variables is indicated by a correlation coefficient value near 1 or −1, whereas a value around zero indicates the absence of a relationship.Similarly, a strong correlation between the parameters is characterised by a positive value in the range of +0.6 to 1.0, while negative values of r indicate an inverse relationship.
The larger the numerical value of the correlation coefficient (r ≥ 0.8), the greater the extent to which correlation holds between the two variables.Table S3 shows the relationship between physicochemical parameters and trace metal elements for the residual wastewater (WW0) of ICP-OES equipment.The Suspended Matter (SM) was found to be highly positively significant to Turbidity, Ca 2+ and colour and negatively significant to Cu 2 + , Mn 2+ , TDS and salinity at p < 0.05.pH was found to be highly positively significant with Fe 2+ and temperature at p < 0.05.Temperature was found to be positively significant with pH and Fe 2+ at p < 0.05 level.Electrical conductivity is highly negatively significant with Fe 2+ whereas TDS is positively significant with salinity and negatively significant with SM, Colour, turbidity and Ca 2+ at p < 0.05.The colour is positively significant with turbidity and Ca 2+ but negatively with Mn 2+ and Cu 2+ .The turbidity is positively significant with Ca 2+ and negatively significant with Mn 2+ at p < 0.05 whereas Mn is positively significant Cu 2+ and Zn 2+ but negatively with Ca 2+ at p < 0.05.At the same level, a positively significant correlation was found between Cu 2+ and Zn 2+ .Without any treatment, salinity is more interrelated with TDS (r2 = + 0.994), SM (r2 = -0.991),turbidity (r2 = -0.991),Colour (r2 = -0.948)and Ca 2+ (r2 = -0.982).pH was found to be strongly interrelated with temperature (r2 = + 0.866) and Fe 2+ (r2 = 1.00).Another strong relation was found between colour with turbidity, Mn 2+ , Ca 2+ and Cu 2+ (r2 = +0.983,-0.925, +0.991 and -0.836).Mn 2+ shows a strong relation with Ca 2+ , Cu 2+ and Zn 2+ (r2 = −0.8660,+0.9820 and +0.8660).Cu 2+ ions was found to correlate strongly with Zn 2+ (r2 = +0.945).
Table S5 shows the relationship between physicochemical parameters and when the wastewater was treated with eggshells.The pH of wastewater WW3 was found to be highly significantly positively correlated with Ca 2+ and Zn 2+ but significant negatively with TDS, Fe 2 + and Salinity at p < 0.05.The temperature was found to be significantly positively correlated with Mn 2+ and Cu 2+ but significantly negative with CE, SM and Turbidity at p < 0.05.The CE was equally found to be significant positively correlated SM and turbidity but significantly negatively correlated with Mn 2+ and Cu 2+ .Salinity was found significant positively with TDS and Fe but significant negative with Ca 2+ and Zn 2+ at p < 0.05.The TDS of the third treatment was found to significantly positive with Fe 2+ but significantly negative with Ca 2 + and Zn 2+ at p < 0.05.The SM was found to be significantly positively correlated to turbidity and significantly negative to Mn 2+ and Cu 2+ at p < 0.05.The colour was found to be significantly negative to Mn 2+ and Cu 2+ at p < 0.05.It was equally observed that Turbidity was negatively significant to Mn 2+ and Cu 2+ at p < 0.05.In the same sample Fe 2+ was found to be significantly negative correlated to Ca 2+ and Zn 2+ at p < 0.05.The value of Mn and Ca 2+ was found to be significantly positively correlated to that of Cu 2+ and Zn 2+ respectively.With the third treatment, pH was found to be more interrelated with both Ca 2+ and Zn 2+ (r2 = +0.956each) and TDS, Fe 2+ and salinity respectively (r2 = −0.974,−0.974 and −0.956).Temperature was found to be strongly interrelated with Mn 2+ (r2 = −0.982),Cu 2+ (r2 = −0.982),EC, SM and Turbidity (at r2 = -1.000,each).Another strong relation was found between EC with SM and Turbidity (at r2 = +1.000each), Mn 2+ and Cu 2+ (with r2 = −0.982each).Salinity shows a strong relation with TDS, Fe 2+ (r2 = −0.866,−0.866), Ca 2+ and Zn 2+ (with r2 = −1.000each).TDS was found to be strongly related to Fe 2+ (r2 = +1.000),Ca 2+ and Zn 2+ (with r2 = −0.866each).SM was found to be strongly interrelated with Turbidity (r2 = +1.000),Mn 2+ and Cu 2+ (r2 = −0.982each).Colour was found to be strongly related to Mn 2+ and Cu 2+ (r2 = −0.866each).Turbidity was found to be strongly interrelated for both Mn 2+ and Cu 2+ (with r2 = −0.982).Fe 2+ was equally found to be strongly interrelated with Ca 2+ and Zn 2+ (with r2 = −0.866each).Mn 2+ was found to be strongly interrelated with Cu 2+ (r2 = +1.000)as well as Ca 2+ and Zn 2+ (each at r2 = +1.000).Positive correlation of pH with Zn 2+ Cu 2+ indicates that the mobility of those trace metal ions decreases when the pH increases due to their fixation on both CCAC and eggshells [52].

Principal components analyses (PCA)
Industrial processes often present huge amounts of process data due to the large number of frequently measured variables.One of the most widely used methods to deal with this problem in industries is PCA.According to Helena et al. (2000) [53], principal components provide information on the most meaningful parameters, which describes a whole data set affording data reduction with minimum loss of original information.In this research work, PCA was applied to summarise the statistical correlation among components in the samples (Table S6).PCA on the correlation matrix resulted in the following biplot for the first two principal components (Figure 3).It was carried out based on the results obtained in the correlation analysis.The first two resultant components from the PCA were conducted on the 13 variables (pH, Temp., EC, Salinity, TDS, Turbidity, Colour, SM, Fe 2+ , Mn 2+ , Ca 2+ , Cu 2+ and Zn 2+ ), explained altogether 91.1 % of the data variability.The first principal component (PC1), 61.95 % from the total variability of the original data, was positively correlated with temp., EC, Salinity and TDS and negatively correlated with pH, SM, turbidity and Ca.In addition, PC1 resulted in data reduction, where only eight variables were required (of the original 13) to explain 62 % of the total variance (Table S6).PC2 explained 29.14 % of the variance and was correlated with Zn 2+ , Fe 2+ , Mn 2+ , Cu 2+ and colour.In this way, PC1 represented the physicochemical component and PC2 corresponded to the mineral component.In addition, three distinct groups were constituted for all the variables (Figure 3).
Group 1, characterised by the initial or raw effluent WW0 is highly influenced by the presence of trace elements viz Zn 2+ , Fe 2+ , Mn 2+ and Cu 2+ suggesting that, the trace metal elements cocktail of the raw effluent is more concentrated than on treated effluent .This could come from sampling waters of anthropogenic activities such as mining as well as standard solutions used in the lab for calibration purposes [54].
Group 2, in which the treatments WW1 and WW2 are found, shows that they are more related to physical parameters like Temperature, EC, TDS and salinity resulting from the geographical location and meteorological conditions such as humidity and incoming solar radiation.
Group 3, characterised by the treatment WW3 related to pH, Colour, turbidity, Ca 2+ and SM in which the dissociation of the CaCO 3 increases not only the pH but also enhances the turbidity and the colour due to the denatured proteins [55,56].

Analysis of variance
Statistical testing was applied by using Fisher's test (F value) for one-way ANOVA to ascertain the variations of each parameter between each treatment.The regression models of the quadratic equation given in table S7 determined that, the regression is very high statistically significant (p-value <0.0001), and the model's F values are 1068.886,62.621, 52.558, 41.717,

Conclusion
This work investigated the effectiveness of activated carbon synthesised from corn cobs and eggshells for reducing the trace metal element concentration and acidity of residual water from lab analyses.The outcome of the research explains that the residual water can be grouped into three categories, in which trace metal elements form a single group with 100 % removal of Zn 2+ , Cu 2+ and Mn 2+ in all the process.All the physicochemical parameters except those not set by FAO and trace metal elements are within the permissible limits of the Food and Agriculture Organization of United Nations.The one-way ANOVA and the Tukey's HSD test suggest significant difference for each parameter at each treatment at p < 0.05.In addition, residual waters of Lab analysis treated using a relatively low-tech filtration successively on two cartridges filled with Corn Cobs Activated Carbon and one cartridge filled with eggshells have shown to be promising for application in industrial wastewater treatment thus, can be adopted by several analytical laboratories for environmental safeguarding and consequently for crop irrigation purposes.However, further studies are needed to investigate the pathogenicity and other physicochemical parameters of this wastewater and it will also be very important to study the reusability of both adsorbents for their potential commercial application and economic feasibility in wastewater treatment.

Figure 1 .
Figure 1.Variation of pH at different treatments.

Figure 2 .
Figure 2. Removal efficiency of trace metal elements on activated carbon and eggshell.

1 Figure 3 .
Figure 3. Biplot showing the projections of the variables in the first two PCs and the distribution of the treatments.

Table 1 .
Average values of physicochemical parameters and trace metal elements in each sample.

Table 2 .
Average values of percentages of trace metal elements adsorbed.