Correction to: The utilization of bentonite enhanced termite mound soil mixture as filter for the treatment of paint industrial effluent

A correction to this paper has been published: https://doi.org/10.1007/s42452-021-04499-3


Introduction
Waste may be referred to as an unwanted material in liquid, solid or gaseous form which is discarded in accordance with standard regulations after it has served its primary purpose. The rise in human population growth across the globe without sustainable control measures had resulted in a vast volume of waste generated per day. The effective management of these generated wastes is a perpetual challenge in both developed and developing countries [1,2]. The significant growth of manufacturing industries in Nigeria has contributed to the high volume of generated effluents which has impacted the environmental resources due to the indiscriminate and unsanctioned discharge of untreated effluents into surface water bodies. Depending on the category of industry, wastewater pollutants may include high levels of biological oxygen demand (BOD 5 ), chemical oxygen demand (COD), total dissolved solids (TDS), total suspended solids (TSS), and toxic heavy metals. Filtration is considered an essential wastewater treatment technique due to its efficiency in the removal of suspended particles and reduction of organic and inorganic pollutants. Ripperger et al. [3] classified filtration processes based on four different criteria, namely location of particle retention, generation of the pressure difference, operation mode, and application. In recent studies, different filter media have been used in the removal pollutants from wastewater. For instance, Shafiquzzaman et al. [4] employed low-cost ceramic filter for urban stormwater treatment, whereas Ajibade et al. [5] and Akosile et al. [6] utilized ceramic filter to enhance the microbial quality of household drinking water. Also, Liu et al. [7] used oyster shell for biological aerated filter medium for municipal wastewater. Lawal et al. [8] studied the treatment of agroprocessing wastewater using ceramic wastewater and Gasemloo et al. [9] used sulphated carboxymethyl cellulose nano-filter for tannery wastewater. Composite clayey soils have been applied as an efficient chemical filter and pollutants removal in recent times [10][11][12][13]. Mounds otherwise known as termitaries are structures built by dissimilar termite species from surrounding soils through the redistribution of soil organic matter and elements in their biomass and organo-mineral constructions. They possess low thermal conductivity, resistance to moisture penetration, comparative compressive strength and mostly found in tropical and subtropical geographical environments [14][15][16][17]. Termitaries have been classified as nuisance to agricultural farm lands and wooden infrastructure because of the space occupied and their destructive nature [18]. They are usually destroyed and turned into wastes. This conventional waste has been utilized as construction materials [19][20][21] and an adsorption material in the decontamination of metal polluted effluent [22]. However, the potentials of mound soil as a filter material in the removal of wastewater pollutants are yet to be analysed and ascertained. There is no literature available on the application of enhanced mound material as an alternative filter system for wastewater management. The presence of high pollutant concentrations in the generated industrial wastewater makes adequate treatment sacrosanct prior to their usual disposal into receiving water bodies. There is a need to comprehensively study the applicability of alternative lowcost materials for wastewater management as the on-site biophysiochemical treatment of these generated effluents could be capital-intensive. Thus, this study investigates the applicability of bentonite enhanced termite mound soil mixture as a filter medium for the treatment of paint wastewater and evaluates its pollutants removal efficiency.

Materials
The major materials used for the research are termite mound soil (MS), bentonite (BC) and paint industrial wastewater (PIWW).
(a) Termite mound soil A reddish brown mound soil was sourced from Ifo, Ogun State within the geographical coordinates of latitude 6° 48′ 39.82″ N and longitude 3° 5′ 58.76″ E (Fig. 1). It possesses a specific gravity of 2.53 and classified as a poorly graded soil with silty clay ( Table 1). The X-ray diffraction analysis revealed quartz (93.76 wt%) as the dominant mineral. The unconfined compressive strength and It is characterized with a specific gravity of 2.37 ( Table 1). The X-Ray diffraction analysis revealed montmorillonite (73.73 wt%) as the dominant mineral. (c) Wastewater PIWW used as a contaminant was a composite sample acquired from a major paint manufacturing company's plant situated in Lagos State (Fig. 3). The characterization of the acquired wastewater is shown in Table 3.

Elemental and mineralogy analysis of the filter materials
The major and trace elements present in the bentonite and termite mound soil were identified through the use of X-ray Fluorescence Spectrometer (EDX 3600B Skyray Instrument, USA). The soil samples were air-dried and sifted (fraction below 2 mm). The X-ray diffraction (XRD) technique was employed to determine the mineralogical phase composition and quantification of the materials.

Performance evaluation framework
The applicability of bentonite enhanced termite mound soil mixture as an alternate filter medium for paint effluent management was assessed with the aid of a well-designed and constructed pilot-scale filtration tank (800 × 800 × 800 mm) with four different sections (400 × 400 × 400 mm) designated as AX, AY, BX and BY, respectively, as shown in Fig. 4. The soil mixtures placed in each section were prepared and proportioned by percentage weight as (100% MS), (5% BC + 95% MS), (10% BC + 90% MS) and (15% BC + 85% MS) for sections AX, AY, BX and BY, respectively. The soil mixtures were compacted with optimum water content in three layers to attain 100 mm thickness with the aid of a hand compactor of 7 kg self-weight and cured for 28 days as described by Tucan et al. [11]. The mixture in each section was subjected to paint wastewater loading for 30 weeks. The content schematic of the tank is illustrated in Fig. 5. Filtrate samples were collected from the leakage outlets of each section in triplicate (Fig. 6) and placed in an ice-cooled insulated cooler and transported to the laboratory. The samples were refrigerated at 4 °C upon arrival at the  laboratory preserve their physicochemical qualities prior to analysis in accordance with APHA, [23] and USEPA, [24]. The quantification and analysis of filtrate samples were obtained after the experimental framework. The performance of the filter was evaluated through the relationship between the characterization of raw PIWW and filtrate samples. The removal efficiencies of the filter were determined by using Eq. (1); where R e is the removal efficiency, C i is the initial concentration of contaminant, and C f is the final concentration of contaminant.

Elemental composition of the filter materials
XRF results for the collected soil samples attest to the existence of the following major (Al, Si, P, S, K, Ca, Ti, V, Fe, W, Nb, Mo, Sn, Sb) and trace (Co, Cr, Cu, Mn, Ni, Pb, Zn) elements. The composition is major and trace elements in the bentonite and termite mound are summarized in

Filtrate quantification and flow rate
The filtrate quantification, flow rate and period of debut droplet from respective filters are presented in Table 3.
Filter AX with 100% termite mound soil (control) has the highest filtrate discharge of 1.1 L with a corresponding flow rate of 11 × 10 −4 LPH while filter BY with bentonite and termite mound ratio of 15:85 has the lowest filtrate discharge and flow rate of 0.2 L and 0.43 × 10 −4 LPH, respectively. The particles of cohesive soils have the tendency to stick to each other due to intermolecular interactions and greater quantity of clay particles produces high liquid limit, as a result, they usually have low permeability [25,26]. The liquid limit of bentonite is more than thrice compared to that of termite mound soil. The low filtrate discharge recorded for filter BY could be attributed to more fines present in bentonite compared to termite mound soil.

Characterization of raw and treated paint industry wastewater.
The characteristics of the raw paint wastewater are presented in Table 4. The total dissolved solids (TDS), biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are 585, 254 and 569 mg/L, respectively. The heavy metals analysis revealed elements such as lead (0.35 mg/L), chromium (0.76 mg/L), copper (1.43 mg/L), cadmium (0.43 mg/L) and nickel (9.45 mg/L). The concentrations of TDS, BOD, COD, copper and nickel were above the permissible limits of NESREA [27]. Similar results were reported by Oladele et al. [28] and Onuegbu et al. [29]. Hence, it's imperative to treat the wastewater prior to its discharge into the environment to forestall the pollution of surface and groundwater. The trends of bentonite content on colour, TSS, Pb, Cr, Cu, Ni, TDS, BOD and COD of the treated samples are shown in Figs. 9 and 10. The strength of pollutants in the filtrate samples generally reduced with the stepped introduction of bentonite. However, the BOD 5 and COD of the treated samples ( Fig. 10) failed to comply with NESREA (BOD 5 ≤ 30 mg/L and COD ≤ 60 mg/L) permissible values for discharge into inland waters. The availability of organic compounds (nitrocellulose, alkyd resins and acrylic/styrene co-polymer) and oxidizable inorganic compounds (pigments and additives) is responsible for the impact on BOD 5 and COD [29].

Filter removal efficiency
The performance of the filters was assessed based on their removal efficiency (RE). Table 4 presents the RE of the filters with respect to their bentonite contents. The RE of colour for filter AX, AY, BX and BY is 7.3%, 12.7%, 16.4% and 20%, respectively. Hu et al. [30] stated that application of bentonite as an adsorbent of basic red dye is largely based on its ability to exchange cations. The best colour RE recorded for filter BY is largely based on the high cation exchange capacity of the bentonite used. The RE of TDS ranged from 19.7 to 51.3% reduction while that of TSS ranged from 97.8 to 98.9% reduction (Fig. 11). Healy [31] reported that bentonite-based hydrophobic media has the capacity to absorb up to 60% of its weigh in organic contaminants. The high reduction in TSS and TDS of the treated samples may be attributed to the capacity of bentonite to adsorb particulate matter on its surface. The RE of copper significantly increased from 5.6% for filter AY to 72.7% reduction for filter BY with the stepped introduction bentonite. The finding is in tandem to that of Cao et al. [ (Fig. 11). Bentonite is characterized with a high specific surface area, tendency to absorb water in the interlayer sites and affinity to adsorb ions from solutions [33,34]. The excellent removal efficiencies of metal ions generally recorded for filter BY could be attributed to its high adsorption capacity and vast specific area.

Conclusion
The investigation on the applicability of bentonite (BC) enhanced termite mound soil (MS) mixture as an alternate filter medium for the treatment of paint industry wastewater revealed the following conclusions.
a. Filter BY with 15% bentonite content is the best filter compared to other filters with lower percentage of bentonite. and Cu. c. The blend of 15%BC + 85% MS can be applied effectively as an alternate filter medium for the treatment of paint industry wastewater. d. The filtration technique can be applied in small paint industry to remove pollutant from their effluent due to its design simplicity, availability of filter materials, cost and treatment efficiencies.

Compliance with ethical standards
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.