Abstract: Background and Objective: High concentration of heavy metals has been discovered in the vegetables in growing area of Kano posing a risk to the soil and its productivity. The feasibility of organic residues such as maize cob and rice husk as adsorbent were evaluated in heavy metal-contaminated soils. The objective of this study was to assess efficacy of using maize cob and rice husk obtained as bio-sorbents in reducing soil solution concentration of heavy metals as well as the influence of contact and residence time on the adsorption of heavy metals: Cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) by different soil fractions (silt-clay and very fine sand). Materials and Methods: Soil samples were collected from five vegetable gardens in Kano and fractionated into silt-clay and very fine sand fractions. Cu, Pb, Zn in the varying concentration 0, 50, 200 and 400 mg kg–1 and 0, 5, 10 and 50 mg kg–1 Cd were added to the soil along with organic residue. This was incubated for a period of 1 and 7 days. Concentration of the heavy metal after the incubation periods was determined at time intervals of 1, 2, 4 and 8 h in a batch experiment. Data were analyzed using two-way ANOVA. Results: Results obtained showed that sorption increased with time, dosage of the organic residue and incubation. Amongst all the residues, maize cob had better sorption efficiency. The adsorption data obtained were perfectly fitted to the Freundlich adsorption model with an R2 value of 0.99 for Cu, Pb and Zn and the sorption process was found to be a cooperative process between Cu, Pb and Zn. For Cd, the R2 was >0.80 with a favorable and normal adsorption process. Conclusion: Heavy metal sorption by the organic residues at all concentrations was very high. A positive relationship between the rate of adsorption and dosage was obtained indicating that the adsorption process was influenced by increasing the dosage of the residue as well as the shaking period. These amendments which are mostly byproducts of agricultural processing can be used as an excellent source of reducing heavy metal availability in contaminated soils.
INTRODUCTION
Heavy metal is a loose term for a group of elements that exhibit metallic properties including the transition elements, some metalloids, actinides and lanthanides1. Agricultural activities such as wastewater irrigation and use of agrochemicals2-7 have led to the excessive release of heavy metals such as Cd, Zn, Cu and Pb into the environment, thus, creating a major global concern related to environmental and human health problems. Very worrisome concentration of Pb, Cd, Cu and Zn has been reported in Kano8-10. These high concentrations affect the soil productivity as well as physiological process in plant such as inhibition of photosynthesis as in the case of Pb uptake. Heavy metals pose a great danger to human health in areas where vegetable is cultivated due to its direct inclusion in food chain11,12.
Various treatment techniques and processes have been used to remove heavy metals from contaminated water and soils13. Among all the approaches proposed, sorption has been considered as one of the most popular methods for economic and efficient purification of heavy metal contaminated soils and water14. Adsorption is one of the physicochemical treatment process effective in removing heavy metals from aqueous solutions and according to Bailey et al.15, an adsorbent can be considered as cheap if it is abundant in nature and requires little processing and is a by-product or waste material from industry. The major advantages of using organic residue as adsorbent in sorption processes as compared to conventional treatment methods are: Low cost, minimization of chemical and/or biological sludge, regeneration of bio-sorbent and no additional nutrient requirement and possibility of metal recovery. Plant wastes are inexpensive as they have very low economic value. Several studies have been conducted on the use of organic residues such as papaya wood16, maize leaf17, teak leaf powder18, lalang (Imperata cylindrica) leaf powder19 and rubber (Hevea brasiliensis) leaf powder20,21 as adsorbent for heavy metal removal in contaminated soil. There is a growing trend to evaluate the feasibility and suitability of natural, viable, renewable and low-cost materials as adsorbent to ameliorate heavy metal pollution. The objectives of the current study were:
| • | To determine the efficacy of using maize cob and rice husk obtained as bio-sorbents in reducing soil solution concentration of heavy metals in polluted soils |
| • | To determine the influence of contact and residence time on the adsorption of heavy metals such as Cd, Zn, Cu and Pb in soils |
MATERIALS AND METHODS
Sampling and sample preparation: Soil samples were collected to a depth of 20 cm from five different vegetable gardens in Gadon-Kaya area of Kano city at a co-ordinate of 11°58’25.040" N and 8°29’46.056" E. Five sub-samples were taken from each vegetable gardens and mixed thoroughly together to obtain a single representative sample. The soil samples were air dried, crushed gently and passed through 2 mm sieve and further separated by sieving into silt- clay and very fine sand fraction.
Two different types of organic residues were used: Rice husk, maize cob and a mixture of the rice husk and maize cob. The rice husk was used in its original form while the maize cob was crushed into fine particles.
Laboratory analysis: Particle size distribution was determined using the principles of Bouyoucous-hydrometer method following chemical dispersion of the soil with calgon solution22. Cation exchange capacity (CEC) was determined by replacing the cations on the exchange sites with 1 M ammonium acetate (NH4OAC) buffered at pH 7 as described by Rhoades23 while exchangeable bases was determined as according to Anderson and Ingram24. Soil pH was measured in both water and CaCl2 at a soil: Water and soil CaCl2 ratio of 1:2.5 using a glass electrode pH meter. Organic carbon (OC) content of the soil was determined by dichromate oxidation method25. Total N was determined using the micro- Kjeldahl digestion method of Bremmer and Mulvaney26 while available P was extracted using the Bray 1 method27.
Adsorption experiment: The soil samples from each fraction i.e. silt- clay and very fine sand were thoroughly mixed with the organic residue in the ratio 1:2, 1:3 and 1:4 (i.e.1 g of soil: 2 g of organic residue, 1 g of soil: 3 g of organic residue and 1 g of soil:4 g of organic residue). The soil sample plus organic residue were then transferred into the incubation vessels and 20 mL each of the varying concentrations of heavy metal (0, 50, 200 and 400 mg of Cu, Pb and Zn per gram of soil while 0, 5, 10 and 50 mg of Cd per gram of soil) were added. The samples were subjected to two different incubation period (1 and 7 days). All samples were replicated 3 times.
After the incubation period, the samples were placed on a mechanical shaker and shaken for a period of 1, 2, 4 and 8 h. Fifteen mililiter of the suspension was taken and filtered into a bottle after each shaking period and the amounts of Cd, Cu, Pb and Zn in the supernatant were determined using the AAS (Buck Scientific VGP 210). The amount of heavy metal sorbed by the soil organic residue mixture was calculated from the difference of the initial and equilibrium heavy metal concentration.
Sorption isotherms: Sorption isotherms were used to partition added heavy metals between soil and solution phase. The adsorption isotherms used are Langmuir, Freundlich and Dubinin-Redushkevich (D-R) models.
Freundlich adsorption isotherm: The Freundlich isotherm28 can be applied for non-ideal sorption on heterogeneous surfaces and multilayer sorption. The Freundlich Eq. is expressed as:
| qe | = | Amount of metal adsorbed (mg g–1) |
| Ce | = | Concentration of the metal at equilibrium (mg L–1) |
| Kf | = | Intercept at zero equilibrium concentration |
| n | = | Adsorption intensity |
The linear form of the above Eq. is given as:
Langmuir adsorption isotherm: The Langmuir adsorption model29,30 is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, with no lateral interaction between the adsorbed molecules. Langmuir model is represented by the following Eq.:
| qe | = | Amount of metal adsorbed (mg g–1) |
| Ce | = | Concentration of the metal at equilibrium (mg L–1) |
| q0 | = | Maximum adsorption capacity (mg g–1 or mol g–1) |
| KL | = | Langmuir constant related to the energy of adsorption (L mg–1 or L mol–1) |
| R1 | = | Seperation factor |
The adsorption parameters can be determined when the above equation is converted into a linear form as:
The main characteristic of the Langmuir equation is that it is based on the assumption that all sites have equal adsorption energies. The value of KL indicates the shape of the isotherm to be either unfavorable (KL>1), linear (KL = 1), favorable (0<KL<1), or irreversible (KL = 0). The RL values between 0 and 1 indicate favorable adsorption31-34.
Dubinin-radushkevich adsorption isotherm: The Dubinin-Radushkevich adsorption isotherm35 is usually used to distinguish physical or chemical adsorption processa33,34,36,37. It is expressed mathematically as:
The linear form given as:
With:
Fitting experimental data to the Dubinin-Radushkevich model34,35,36 provides a value of an absolute mean free energy of adsorption |E| in J mol-1:
| qe | = | Amount of metal adsorbed (mg g–1) |
| Ce | = | Concentration of the metal at equilibrium (mg L–1) |
| qmax | = | Maximum adsorption capacity (mg g–1 or mol g–1) |
| KDR | = | Dubinin-Radushkevich constant (mol2 J–2) |
| ε | = | Polanyi potential related to the equilibrium concentration |
| R | = | Universal gas constant |
| T | = | Absolute temperature |
| |E| | = | Absolute mean free energy of adsorption |
The value of mean sorption energy gives information about chemical and physical sorption. The |E| values ranging from 1-8 kJ mol–1 indicates physical sorption mechanism (van der Walls interactions) and values ranging from 9-16 kJ mol–1 indicates chemical sorption (ionic or covalent bonding). Values between 8 and 9 kJ mol–1 show a possibility of both physical and chemical adsorption processes33,34,36,38,39.
Data analysis: Data were analyzed using two-way ANOVA for complete randomized design (CRD)40 to determine the difference in removal efficiency of the heavy metals by the adsorbents and for comparison of the different treatments mean. The analysis was done using SAS version 9.040. The goodness of fit of each graph was used as model confirmation for data. Significant means were separated using SNK at 5% probability level.
RESULTS AND DISCUSSION
Physicochemical properties of soil and organic residue: The physicochemical properties of the soil and the organic residues are shown in Table 1 and 2. The soil is a sandy loam soil and has slightly acidic pH in water and moderately acidic in CaCl2. It is low in organic matter and cation exchange capacity (CEC) with average values of exchangeable bases and available P. Amongst the organic residues, maize cob had the highest electrical conductivity (EC) of 1.74 dS m–1. Of all the metals, Zn had the highest total content and the highest availability in both soil fraction and organic residue. Generally, maize cob had the lowest availability of all the metals.
Sorption efficiency of metal ions on soils amended with organic residue: The adsorption efficiency of the heavy metals with the different organic residue for the clay-silt fraction is presented in Table 3. A significant difference (p<0.05) was observed in the sorption of Cu, Cd and Pb with sorption increasing with concentration. However, adsorption of Zn was observed to decrease at high concentration. Interaction between concentration and type of residue was significant (p<0.01) for all the metals except Zn. Significant differences (p< 0.05) were observed in the effects of organic residues for adsorption of Zn and Pb.
Incubation had a positive influence on sorption as fewer metal ions were available in solution after 7 days as compared to incubating for a day. Shaking period was significantly (p<0.05) different in all the metals with the exception of Zn which had no significant difference in adsorption of heavy metals between shaking for 2 and 4 h.
| Table 1: | Characterization of the soil of used for the experiment |
| S-C: Silt clay fraction, VFS: Very fine sand | |
| Table 2: | Characterization of the organic residue used for the experiment |
| MC: Maize cob, RH: Rice husk and MR: Maize cob plus rice husk | |
The interaction between the different concentrations of heavy metals with the different types of organic residue used for Cd, Cu and Pb were highly significant (p<0.01).
An overview of the adsorption capacity of Cd, Cu, Pb and Zn by the very fine sand fraction is presented in Table 4. A significant difference (p<0.05) between all the concentration and amount adsorbed was observed.
| Table 3: | Sorption efficiency of Cd, Cu, Pb and Zn by organic residues in a silt-clay soil fractions |
HM: Heavy metal concentration, R: Residue, IP: Incubation period, SP: Shaking period. Means followed by different letter(s) are significantly different at 5% level of probability, *,**Significant at 5 and 1% level of probability, respectively, NS: Not significant | |
Significant interactions (p<0.01) effect were observed between concentration and residues, concentration and dosage of residue, concentration and shaking and also concentration and incubation period for all the metals except Cu (Table 3). There was a significant difference (p<0.05) among the entire residue for the sorption of Cd and Zn. Incubation periods showed significant difference (p<0.05) amongst all the heavy metals with the exception of Cu. Shaking period of all the metals was also significantly different (Table 4).
Sorption isotherms: The results of fitting the experimental data to Freundlich adsorption model are presented in Table 5. The model had a good fit for the sorption for Cu, Pb and Zn. Adsorption intensity (1/n) was <1 in Cd and >1 in Cu, Pb and Zn. The Freundlich constant (Kf) value observed in Cd ranged between 0.26-0.29, while the value observed in the rest of the metals was less than 0.1. From Table 5, the Langmuir model was a good fit for Cu, Pb and Zn, however, Cd adsorption did not fit the adsorption model with R2<0.115.
| Table 4: | Sorption efficiency of Cd, Cu, Pb and Zn by organic residues in a very fine sand fraction |
HM: Heavy metal concentration, R: Residue, IP: Incubation period, SP: Shaking period. Means followed by the different letter(s) are significantly different at 5% level of probability. *,**Significant at 5 and 1% level of probability, respectively.NS: Not significant | |
In the very fine sand fraction the model had a good fit for the sorption of Cu, Pb and Zn. The R2 value observed for Cd was also high between 0.797-0.830. The value of the adsorption intensity (1/n) was <1 in Cd. For the rest of the metals, 1/n>1. The Freundlich constant (Kf) value in Cd ranged from 0.30-0.31 and less than 0.1 for the rest of the metals.
Use of the Langmuir model for sorption of Pb, Cu and Zn gave a good fit and it was observed that Cd adsorption did not fit the adsorption model well (R2<0.115). Langmuir constant (k1) was high for the adsorption of Cd (0.071-0.072) when compared to that of the other metals (0.000-0.004). Amount of metal sorbed per min (qe) was high for Cu, Pb and Zn while the qe value obtained for Cd was <0.01.
An overview of the separation factor (RL) of all the heavy metals is presented in Table 6. The RL value in Cd was observed to decrease with increase in the concentration used and the same trend was observed in Zn. In the sorption of Cu by rice husk and maize plus rice husk, concentration had no influenced on the RL value, has the value remained the same regardless of the change in concentration. The same behavior was observed in the sorption of Pb by rice husk. The Dubinin-Raduskevich model was a poor fit as shown (Table 7).
| Table 5: | Freundlich and langmuir adsorption isotherm constants for adsorption of metal ions on different organic residues |
| Table 6: | Langmuir RL value for the different concentration of heavy metals amended with organic residue |
Effect of concentration of heavy metal on metal sorption: The increasing rate of adsorption of all the metals with rate of concentration is related to the retention of the metals ions occurring through an initial rapid sorption onto the external surface of the soil and organic residue, which was then followed by slow diffusion into the soil and organic residue pores.
| Table 7: | Dubunin-Radushkevich adsorption isotherm constants for adsorption of metal ions on different organic residue |
Similar observations were also made by Roth et al.39, Selim et al.41, Bruemmer et al.42 and Kyzas43 and Kyzas et al.44 although an opposing result was obtained by Abdel Salam et al.45 and Attia et al.46 in which sorption was observed to decrease with increasing concentration. Liu47 also proposed that adsorption process may be dependent on the availability of adsorption sites on the surface of adsorbent rather than adsorbate concentration in the bulk solution. However, Zaragoza et al.48 was of the opinion that the chemical and physical properties of the residue as well as its decomposition rate might have an influence on the process.
In the silt-clay fraction, the rate of Cu sorption was higher than all other heavy metals even at a low initial concentration indicating that metal adsorption was influenced by ion characteristic as well as presence of clay minerals39,47. Since all the ions are divalent cations at the same original solution levels, a correlation between ionic size and adsorption selectivity may be expected. The sequence for the rate of sorption was observed as follows: Cu>Pb>Zn>Cd. Higher selectivity for Cu was reported by Hegazi49, Kyzas43 and Wu et al.50 While contrasting results were observed in the study of Elliott et al.51 and Pagnanelli et al.52 where Pb adsorption was highest while Abdel Salam et al.45 and Oh and Tshabalala53 reported an increased affinity for Zn. For the ion exchange process, the strength with which cations of equal charge are held is inversely proportional to the unhydrated radii51,52. Thus, the predicted order of selectivity based on unhydrated radii is Pb2+ (0.120 nm)>Cd2+ (0.097 nm)>Zn2+ (0.074 nm)>Cu2+ (0.072 nm)42,51, 52. Although the unhydrated radii of Cu2+ and Zn2+ are similar, the complexation of Cu2+ with soil is more stable than Zn2+. Therefore, Cu2+ is more readily adsorbed onto soil surface and occupies more adsorption sites than the other ions.
However, in the very fine sand fraction, it was discovered that there is a correlation between ionic size and adsorption selectivity as the rate of Pb sorption was the highest. Brummer et al.42, Pagnanelli et al.52 and Yang et al.54 predicted an order of selectivity for divalent unhydrated radii as Pb2+ (0.120 nm)>Cd2+ (0.097 nm)>Zn2+ (0.074 nm)>Cu2+ (0.072 nm) though the selectivity sequences observed in this soil fraction was Pb>Cu>Zn>Cd. This result was supported by previous findings of Elliott et al.51, Appel and Ma55 and Pagnanelli et al.52 thus indicating that the unhydrated radius may serve as a predictive index of metal adsorption.
Effect of dosage of organic residue on sorption of Cd, Cu, Pb and Zn: The increased adsorption of Cd, Cu, Pb and Zn observed irrespective of metal concentration can be attributed to the availability of larger surface area for adsorption with increase in the amount of adsorbate. This is in consonant with the study of Al-Anber and Al-Anber56, who observed that using 5 g of olive cake had better efficiency of 77% over 1 g which has an efficiency of only 42%. Vieira et al.57 also observed an increase in heavy metal removal percentage by clay as the amount of clay used increases. Similar results were observed by Hegazi49.
Effect of time on sorption of Cd, Cu, Pb and Zn: The rate of sorption of Cd, Cu, Pb and Zn ions onto different organic residue was observed to be very fast in the 1st h with a gradual increase in sorption till after the 8 h. This may be abundance of adsorption sites on the surface of the different residues which allowed easy interaction of heavy metal ions with the adsorption sites. Afterwards, the adsorption sites were almost saturated and the concentrations of heavy metal ions had become low which resulted in the decreased adsorption rate. Similar observation was also made by Salam et al.45, Kyzas43, El-Kamash58 and Wu et al.50. Of importance is the fact that equilibrium was attained much later in this work when compared with Kyzas et al.44, Kyzas43, Hegazi49 and Wu et al.50 where the contact time was less than 4 h. However, the rate of Cd sorption increased with increasing shaking time and the reduced rate of sorption may be as a result of competition between the different metals as Cd, Cu and Zn are divalent elements hence correlation between ionic size and adsorption selectivity may be expected as suggested by Yang et al.54.
Effect of soil fraction on adsorption of Cd, Cu, Pb and Zn: The adsorption of metals was expected to be high in clay and silt fraction due to the presence of clay minerals. This observation is accorded with the fact that these materials have numerous adsorption sites due to their high specific surface areas. The clay and silt fraction sorbed more Cu, Pb and Zn than the very fine sand soil fraction. Spark and Swift59, Zachara and Smith60 and Spark et al.61 reported that adsorption capacity is linked to the surface site density and that the presence of clay minerals enhances the adsorption capacity. A general tendency of adsorption capacities can therefore be concluded that the adsorption capacity of the very fine sand fraction is not as high as compared to the clay and silt fraction due to the absence of clay minerals. The study of Roth et al.39 and Kim and Fergusson62 is in agreement with these results. However, the present study is not in agreement with both Kim and Fergusson62 and Roth et al.39 on the adsorption of Cd, because a reversal of the trend was observed in Cd as more Cd ions were sorbed by the very fine sand fraction as compared to the clay and silt soil size fraction which may be based on the argument of Yang et al.54 on the occurring competition between the various ions on the adsorptive sites of the adsorbate.
Effect of organic residue on sorption of Cd, Cu, Pb and Zn: Organic residue had a great impact on sorption in very fine sand soil fraction as the rate of adsorption which is usually generally low was high due to the presence of the residues. Thus, organic residue favors sorption due to the release of humic substances, which enhance the sorption of positive ions by making the surface more negative. This collaborates the findings of Masset et al.63 who observed that humic acid had an influence on sorption of Co (II), Sr (II) and Se (IV), Wu et al.50 and Zaragoza et al.48 The percentage of sorption for Pb and Cu were high when rice husk and maize cob were used as an adsorbent, whereas only maize cob gave good sorption efficiency for Zn and Cd.
The adsorption capacity of rice on the sorption of Pb and Cu were 11.14 and 10.77 mg g–1 respectively which is low as compared to adsorption capacity obtained by Wong et al.64 which was 108 and 29 mg g–1 for both metals respectively. However the rice husk used in their study was modified with esterified tartaric acid. Tarley et al.65, obtained an adsorption capacity of 4 mg g–1 for Cd, which is lower than 211 mg g–1 obtained in the present. The sorption efficiency for all the organic residues obtained was found to be similar to that of Suemitsu et al.66, Hegazi49 and Zaragoza et al.48 where efficiency of over 80% was recorded. Removal efficiency of Cu by maize cobs was not affected by the presence of Pb and Zn as observed by Khan and Wahab67.
Describing heavy metal sorption with Freundlich and Langmuir adsorption isotherms: The sorption data obtained was fitted into Langmuir, Freundlich and Dubunin- Radushkevich isotherms out of which Freundlich adsorption model was found to have the best fit for all the heavy metals.
Freundlich adsorption model was found to be favorable for the sorption of Cd as the value of 1/n was <1 in both fractions of the soil irrespective of the type of organic residue used. This value according to Mohan and Karthikeyan68 indicates that the sorption process was normal and sorption process is favorable since the value of n is >169. The value of 1/n observed for Cu, Pb and Zn was greater than 1. Hameed et al.70 and Dada et al.71 reported that adsorption process with 1/n> is a cooperative adsorption process. Similar fit with the Freundlich data were obtained by Wu et al.50 and Roth et al.39 however Kyzas43 and Salam et al.45 had better results with Langmuir model.
From the results obtained, it was observed that the Langmuir isotherm fits quite well with the experimental data of Cu and Pb with a high R2 value which is in agreement with the results of the Freundlich isotherm. This may be due to the presence and homogenous distribution of active sites on the organic residues which also agrees with the study of Dada et al.71 on equilibrium studies using adsorption isotherms. The value of the KL obtained indicates that the isotherm is irreversible32 and since the RL lies between 0 and 1, the sorption is said to be favorable and is in accordance with Eastoe and Dalton30, Zheng et al.34, 72, Saltah et al.33. However, the sorption of Zn in the very fine sand fraction is in consonant with these authors. The isotherm was observed to show a very poor fit for Cd ions.
CONCLUSION
The suitability of each residue as a potential adsorbent was observed, however, the sorption efficiency of maize cob was higher than that of rice husk or a mixture of maize cob and rice husk with a positive relationship between the rate of adsorption and dosage obtained. A perfect fit was observed for the Freundlich adsorption model with the sorption process of Cu, Pb and Zn found to be a cooperative process (1/n>1) while Cd was found to be favorable and normal adsorption process. These residues can be used as excellent source of heavy metal removal in contaminated soils.
SIGNIFICANCE STATEMENT
Organic residues such as maize cobs, rice husk and mixture of both as adsorbents are capable of reducing heavy metal contamination. The adsorption of Cu, Pb and Zn is competitive on the adsorbent while Cd followed normal adsorption process. The possibility of improving adsorption by incubation and shaking was also explored.
ACKNOWLEDGMENT
The author wishes to thank the Staff of the Department of Soil Science Laboratory, Bayero University Kano, for their assistance in the process of carrying out this study and the Centre for Dryland Agriculture, Bayero University Kano, for funding the publication of this study.