On the dominance of Pb during competitive biosorption from multi-metal systems: A review

Figures & data

Figure 1. Number of papers appearing with “lead” or “Pb” and “biosorption” in the topic listed in the Scopus database for period 2000–2017. Database searched on 23.10.18.

Figure 2. Number of citations of papers appearing with “lead” or “Pb” and “biosorption” in the topic listed in the Scopus database for period 2000–2017. (Total number of articles appearing = 844: database searched on 23.10.18; sum of the times cited = 27, 410; average citations per article = 32.5; h index = 16; articles with zero (0) citations to date = 131).

Table 1. The top 10 most cited articles in the Scopus database appearing with “lead” or “Pb” and “biosorption” in the topic (out of a total of 844 articles appearing: database searched 23.10.18)

Article	Number of times article cited	Summary of conclusions
Fourest and Roux (1992)	599	Adsorption of metal ions from mono-component systems by Rhizopus arrhizus followed the order: Pb(55.6 mg/g)>Cd(26.8 mg/g)>Zn(13.5 mg/g)>Ni(18.7 mg/g). Trend indicates that biosorbent had the highest affinity for Pb.
Kapoor and Viraraghavan (1995)	586	Fungal biosorption generally depended on pH, metal ion concentration, biomass concentration, and presence of various ligands in solution.
Gupta and Rastogi (2008)	384	Maximum adsorption capacity for the removal of Pb from single metal solution using Spirogyra sp. was found to be 140 mg/g. Basing on this value, it was concluded that the adsorbent was effective in Pb adsorption.
Chang, Law, and Chang (1997)	364	Adsorption of metals from mono-component systems by P. aeruginosa followed the order: Pb(110 mg/g)>Cd(58 mg/g)>Cu(23 mg/g). Increase in Hg concentration (from 0 to 50 mg Hg^2+/litre) in the growth media had no significant effect on metal adsorption.
Holan and Volesky (1994)	356	Adsorption of Pb and Ni by marine algae followed the order: Pb(370 mg/g)>Ni(74 mg/g). The lower adsorption capacity for Ni was explained in terms of its lower selectivity coefficient for alginates when compared to Pb.
Saeed et al. (2005)	349	Adsorption capacities for the removal of metals by crop milling waste followed the order: Pb(49.97 mg/g)>Cd(39.99 mg/g)>Zn(33.81 mg/g)>Cu(25.73 mg/g)>Ni(19.56 mg/g). The presence of Pb did not affect the efficiency of removal of other metals in binary and ternary solutions.
Krishnani, Meng, Christodoulatos, and Boddu (2008)	331	Adsorption capacities for the removal of metals followed the order: Pb(58.02 mg/g)>Hg(36.11 mg/g)>Cd(16.75 mg/g)>Cu(10.93 mg/g)>Mn(8.30 mg/g)>Zn(8.11 mg/g)>Ni(5.52 mg/g)>Co(4.54 mg/g)
Fiol et al. (2006)	330	Removal of metals from single component systems using olive stone waste followed the order: Pb(9.26 mg/g)>Cd(7.74 mg/g)>Ni(2.13 mg/g)>Cu(2.03 mg/g) For binary systems: Pb(10.90 mg/g)>Cu(3.22 mg/g) Pb(10.24 mg/g)>Ni(3.10 mg/g) Pb(9.84 mg/g)>Cd(7.61 mg/g) Comparison of adsorption capacities for single and binary sorption systems shows that sorption of Cd was not significantly affected by the presence of Pb, and sorption of Cu and Ni was actually enhanced in the presence of Pb.
Yan and Viraraghavan (2001)	325	For single metal adsorption using M. rouxii, the adsorption capacities were: Pb(4.06 mg/g)>Cd(3.76 mg/g)>Zn(1.36 mg/g)>Ni(0.36 mg/g)
Sari and Tuzen (2009)	295	Maximum adsorption capacities of A. rubescens for Pb and Cd was found to be: Pb(38.4 mg/g)>Cd(27.3 mg/g).

Table 2. Classification of initial concentration variations

Variation of initial concentration	Summary of conclusions	References
Condition (I) Simultaneously varying the initial concentrations $(C_o$) of the pollutants $X_1$ and $X_2$, where $C_{o,X_1}=C_{o,X_2}$	Binary sorption capacities obtained using pine cone shell biosorbent followed the order: Pb(17.41 mg/g)>Cu(6.52 mg/g). However, the single-metal uptake capacities of the biomass for copper and lead were slightly inhibited by the presence of either metal ion.	Martín-Lara, Blázquez, Calero, Almendros, and Ronda (2016)
	Chitosan-pyromellitic dianhydride (PMDA)-modified biochar had strong selective adsorption of Cu(II) compared to Cd(II) and Pb(II).	Deng et al. (2017)
	The affinity between metals and biosorbent calculated by the separation factor in binary metals systems followed the order: Pb>Cd>Zn Pseudoalteromonas sp. SCSE709-6 was a competitive biosorbent for metals from aqueous solutions in single system (maximum adsorption capacity	Jiang et al. (2017)
	Chitosan-based nano composite showed higher adsorption of Pb(II) than Cd(II). Decrease in adsorption of Cd(II) in the presence of Pb(II) was more than decrease in Pb(II) adsorption in the presence of Cd(II).	Maity and Ray (2018)
	Calcium alginate, algal biomass and algal/glutaraldehyde-crosslinked polyethyleneimine showed a greater affinity for Pb(II) compared to Cu(II) regardless of the molar ratio of competitor ions	Wang et al. (2017)
	Myriophyllum spicatum had no preference for Cu(II) and Pb(II) in spite of their differences in physicochemical properties.	Yan et al. (2010)
	Maximum adsorption capacities for single metal removal of Cu(II) and Pb(II) using activated carbon from Eucalyptus camaldulensis Dehn bark were 109.8 and 28.6 mg/g, respectively. Trend also observed for binary metal systems.	Kongsuwan, Patnukao, and Pavasant (2009)
	The affinity of Pb(II) for almond shell was higher than that of Cu(II), with maximum adsorption capacities of 13.7 and 9.0 mg/g obtained, respectively	Ronda et al. (2013)
	Compared to single metal sorption using Pithophora oedogonia, the adsorption capacities for the removal of Cu(II) and Pb(II) were reduced in binary system. However, the adsorbent showed a greater affinity to sorb Cu(II) than Pb(II) from binary solution.	Kumar et al. (2008)
	Affinity of Pb(II) for Industrial chili seeds (Capsicum annuum) waste was more than 5 times higher than that of Cd(II).	Medellin-Castillo et al. (2017)
	The presence of Cu(II) adversely affected Pb(II) adsorption onto Chlorella vulgaris, whereas Zn(II) ions seemed to have negligible effect on the process.	El-Naas et al. (2007)
	Ni(II) and Zn(II) had lower interference effect on Pb(II) sorption by Arthrospira (Spirulina) platensis and Chlorella vulgaris than the contrary, hence biosorbents had greater preference to Pb(II).	Rodrigues et al. (2012)
	The affinity order of calcium-saturated anaerobic sludge biosorbent was established as: Pb > Cu > Ni > Cd. The single-metal sorption for Pb was slightly inhibited by the presence of Cu and Cd cations (by 6%) and by the presence of nickel (by 11%).	Hawari and Mulligan (2007)
Condition (II) Simultaneously varying the initial concentrations of both components with $C_{o,X_1}\ne C_{o,X_2}$	As summarized under Condition (I) above	El-Naas et al., 2007; Yan et al., 2010; Ronda et al., 2013; Wang et al., 2017
Condition (III) Varying the initial concentration of one of the components whilst keeping the other constant	Adsorption capacities were 11.76 and 27.47 mg/g, for Cd(II) and Pb(II), respectively. Competition, interaction or displacement effect observed between Cd(II) and Pb(II) ions.	Módenes, Espinoza-Quiñones, Colombo, Geraldi, & Trigueros, 2015
	As described above under Condition (I)	Wang et al., 2017

Table 3. Binary sorption: Pb/Cu system

Adsorbent	Solution component	Metal ions	$q_m$ (mg/g)	${\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{X}}^{\mathbf{\prime }}/{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{X}}^{\mathit{o}}$	${\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{P}\mathit{b}}^{\mathit{o}}/{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{C}\mathit{u}}^{\mathit{o}}$	${\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{P}\mathit{b}}^{\mathbf{\prime }}/{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{C}\mathit{u}}^{\mathbf{\prime }}$	Reference
Chitosan based nano-composite	Single	Pb(II) Cd(II)	357.4 341.6		1.046		Maity and Ray (2018)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Pb/Cd) Cd(II) (Pb/Cd)	313.7 303.6	0.8778 0.8888		1.033
TEPA modified chitosan/CoFe2O4	Single	Cu(II) Pb(II)	168.067 228.311		1.358		Fan et al. (2017)
	Binary ($C_o$ ratio 1:1)	Cu(II) (Cu/Pb) Pb(II) (Cu/Pb)	139.665 160.256	0.8310 0.7020		1.147
Alginate	Single	Pb(II) Cu(II)	414.4 113.1		3.664		Wang et al. (2017)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Cu/Pb) Cu(II) (Cu/Pb)	385.4-	0.9300-		-
Pine cone shell	Single	Cu(II) Pb(II)	6.808 25.24		3.707		Martín-Lara et al. (2016)
	Binary ($C_o$ ratio 1:1)	Cu(II) (Cu/Pb) Pb(II) (Cu/Pb)	6.522 17.407	0.958 0.690		2.669
Alginate-immobilized Trichoderma asperellum	Single	Pb(II) Cu(II)	89.59 71.99		1.2445		Chew and Ting (2016)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Cu/Pb) Cu(II) (Cu/Pb)	112.70 31.53	1.2580 0.4380		3.5744
Cabbage waste	Single	Cu(II) Pb(II)	10.315 60.568		5.872		Hossain et al. (2014)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Pb/Cu) Cu(II) (Cu/Pb)	60.311 9.447	0.9958 0.9159		6.384
Almond shell	Single	Pb(II) Cu(II)	26.546 9.438		2.8126		Ronda et al. (2013)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Pb/Cu) Cu(II) (Cu/Pb)	13.712 8.952	0.5165 0.9485		1.5317
Potassium hydroxide treated pine cone powder	Single	Pb(II) Cu(II)	32.26 26.32		1.2257		Ofomaja et al., (2010a)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Pb/Cu) Cu(II) (Cu/Pb)\	30.12 22.68	0.9337 0.8617		1.3280
Valonia tannin resin	Single	Pb(II) Cu(II)	98.55 39.21		2.513		Şengil and Özacar (2009)
	Binary ($C_o$ ratio 1:2) ($C_o$ ratio 1:1)	Pb(II) (Cu/Pb) Cu(II) (Cu/Pb)	78.2 31.1	0.7935 0.7932		2.514
Mansonia wood sawdust	Single component	Pb(II) Cu(II)	51.81 42.37		1.222		Ofomaja et al., (2010b)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Cu/Pb) Cu(II) (Cu/Pb)	61.034 48.84	0.8356 0.8676		1.250
Myriophyllum spicatum	Single	Pb(II) Cu(II)	51.72-	0.8928			Yan et al. (2010)
	Binary ($C_o$ ratio 1:8)	Pb(II) (Cu/Pb) Cu(II) (Cu/Pb)	46.18-
Aspergillus flavus	Single	Pb(II) Cu(II)	11.30 9.98		1.1323		Akar and Tunali (2006)
	Binary ($C_o$ ratio 1:1)	Pb(II) (Cu/Pb) Cu(II) (Cu/Pb)	6.41 3.04	0.5672 0.3046		2.1086

Table 4. Comparison of total equilibrium sorption capacity for selected biosorbents-Pb/Cu system

Biosorbent	$\mathbf{(}{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{P}\mathit{b}}^{\mathit{o}}\mathbf{+}{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{C}\mathit{u}}^{\mathit{o}}\mathbf{)}$	$\mathbf{(}{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{P}\mathit{b}}^{\mathbf{\prime }}\mathbf{+}{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{C}\mathit{u}}^{\mathbf{\prime }}\mathbf{)}$	$\left[\frac{{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{P}\mathit{b}}^{\mathbf{\prime }}}{\left({\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{P}\mathit{b}}^{\mathbf{\prime }}+{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{C}\mathit{u}}^{\mathbf{\prime }}\right)}\right]$ x 100%	References
TEPA modified chitosan/CoFe2O4	396.378	299.921	76%	Fan et al. (2017)
Pine cone shell	32.048	23.929	73%	Martín-Lara et al. (2016)
Alginate-immobilized Trichoderma asperellum	161.58	144.23	78%	Chew and Ting (2016)
Cabbage waste	70.883	69.758	86%	Hossain et al. (2014)
Almond shell	35.984	22.664	61%	Ronda et al. (2013)
Potassium hydroxide treated pine cone powder	58.58	52.80	57%	Ofomaja et al. (2010b)
Valonia tannin resin	137.76	109.3	72%	Şengil and Özacar (2009)
Mansonia wood sawdust	94.18	109.874	56%	Ofomaja et al., (2010b)
Aspergillus flavus	21.28	9.45	68%	Akar and Tunali (2006)

Table 5. Ternary sorption

Adsorbent	Solution component	Metal ions	${\mathit{q}}_{\mathit{m}}$ (mg/g)	${\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{X}}^{\mathbf{\prime }}/{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{X}}^{\mathit{o}}$	$\left[\frac{{\mathit{q}}_{\mathit{m}\mathbf{,}{\mathit{X}}_{\mathbf{1}}}^{\mathbf{\prime }}}{\left({\mathit{q}}_{\mathit{m}\mathbf{,}{\mathit{X}}_{\mathbf{1}}}^{\mathbf{\prime }}+{\mathit{q}}_{\mathit{m}\mathbf{,}\mathbf{2}}^{\mathbf{\prime }}+{\mathit{q}}_{\mathit{m}\mathbf{,}{\mathit{X}}_{\mathbf{3}}}^{\mathbf{\prime }}\right)}\right]\mathbf{x}100\mathrm{\%}$	Reference
Date seed biochar	Single component	Pb(II) Cu(II) Ni(II)	148.77 26.75 19.54			Mahdi, Yu, and El Hanandeh (2018)
	Ternary component ($C_o$ ratio 1:1:1)	Pb(II) (Pb/Cu/Ni) Cu(II) (Pb/Cu/Ni) Ni(II) (Pb/Cu/Ni)	43.51 6.61 10.33	0.292 0.247 0.529	72.0% 10.9% 17.1%
Aluminate treated Casuarina equisetifolia leaves	Single component	Pb(II) Cu(II) Ni(II)	28.60 44.59 39.93			Rao and Khatoon (2017)
	Ternary component ($C_o$ ratio 1:1:1)	Pb(II) (Pb/Cu/Ni) Cu(II) (Pb/Cu/Ni) Ni(II) (Pb/Cu/Ni)	21.73 13.00 7.80	0.76 0.29 0.20	51.1% 30.6% 18.3%
Cabbage Leaves Powder	Single component	Pb(II) Cu(II) Cd(II)	6.31 5.75 5.07			Kamar, Nechifor, Nechifor, Al-Musawi, and Mohammed (2016).
	Ternary component ($C_o$ ratio 1:1:1)	Pb(II) (Pb/Cu/Cd) Cu(II) (Pb/Cu/Cd) Cd(II) (Pb/Cu/Cd)	4.54 3.35 2.48	0.719 0.583 0.489	43.8% 32.3% 23.9%
Exiguobacterium sp.	Single component	Pb(II) Cd(II) Zn(II)	19.5 11.2 2.55			Park and Chon (2016)
	Ternary component ($C_o$ ratio 1:1:1)	Pb(II) (Cd/Pb/Zn) Cd(II) (Cd/Pb/Zn) Zn(II) (Cd/Pb/Zn)	8.08 3.15 0.915	0.414 0.281 0.359	66.5% 25.9% 7.6%
Cabbage waste	Single component	Pb(II) Cu(II) Zn(II) Cd(II)	60.57 10.32 8.97 20.57			Hossain et al. (2014)
	Ternary component ($C_o$ ratio 1:1:1)	Pb(II) (Cd/Pb/Cu) Cd(II) (Cd/Pb/Cu) Cu(II) (Cd/Pb/Cu)	40.963 12.264 8.785	0.6763 0.5962 0.8513	66.1% 19.8% 14.1%
		Pb(II) (Cd/Pb/Zn) Cd(II) (Cd/Pb/Zn) Zn(II) (Cd/Pb/Zn)	50.22 7.587 1.828	0.8291 0.3689 0.2038	84.21% 12.72% 3.06%
		Pb(II) (Cu/Pb/Zn) Cu(II) (Cu/Pb/Zn) Zn(II) (Cu/Pb/Zn)	22.80 8.194 6.380	0.3765 0.7940 0.7113	61.0% 21.9% 17.1%
Heterogeneous cultures comprising of dried anaerobic bacteria, yeast (fungi), and protozoa	Single component	Pb(II) Cr(III) Cd(II)	54.92 34.78 29.99			Sulaymon, Ebrahim, and M-Ridha (2014)
	Ternary component $C_o$ ratio 1:1:1	Pb(II) (Pb/Cr/Cd) Cr(III) (Pb/Cr/Cd) Cd(II) (Pb/Cr/Cd)	15.68 13.86 4.18	0.2855 0.3985 0.1394	46.5% 41.1% 12.4%
Xanthate-modified magnetic chitosan	Single component	Pb(II) Cu(II) Zn(II)	76.9 34.5 20.8			Zhu, Hu, and Wang (2012)
	Ternary component $C_o$ ratio 1:1:1	Pb(II) (Pb/Cu/Zn) Cu(II) (Pb/Cu/Zn) Zn(II) (Pb/Cu/Zn)	28.6 26.3 6.93	0.372 0.762 0.333	46.3% 42.5% 11.2%
Valonia tannin resin	Single component	Pb(II) Cu(II) Zn(II)	98.55 24.5 16.5			Şengil and Özacar (2009)
	Ternary component $C_o$ ratio 20:5:4	Pb(Pb/Cu/Zn) Cu(Pb/Cu/Zn) Zn(Pb/Cu/Zn)	70 19 0.5	0.7103 0.7755 0.0303	78.2% 21.2% 0.6%
Eicchornia crassipes	Single component	Pb(II) Cd(II) Zn(II)	26.32 12.59 12.55			Mahamadi and Nharingo (2010)
	Ternary component $C_o$ ratio 1:1:1	Pb(Pb/Zn/Cd) Zn(Pb/Zn/Cd) Cd(Pb/Zn/Cd)	14.31 3.66 3.04	0.5437 0.2916 0.2415	68.1% 17.4% 14.5%

Table 6. Trends in adsorption capacities for monocomponent and ternary systems

Adsorbent	Metal ions	Trend in monocomponent system	Trend in ternary system	Reference
Date seed biochar	Pb(II) Cu(II) Ni(II)	Pb>Cu>Ni	Pb>Ni>Cu	Mahdi et al. (2018)
Casuarina equisetifolia	Pb(II) Cu(II) Ni(II)	Cu>Ni>Pb	Pb>Cu>Ni	Rao and Khatoon (2017)
Cabbage Leaves Powder	Pb(II) Cu(II) Cd(II)	Pb>Cu>Cd	Pb>Cu>Cd	Kamar et al. (2016).
Cabbage waste	Pb(II) Cu(II) Cd(II)	Pb>Cd>Cu	Pb>Cd>Cu	Hossain et al. (2014)
	Pb(II) Zn(II) Cd(II)	Pb>Cd>Zn	Pb>Cd>Zn
	Pb(II) Cu(II) Zn(II)	Pb>Cu>Zn	Pb>Cu>Zn
Heterogeneous cultures comprising of dried anaerobic bacteria, yeast (fungi), and protozoa	Pb(II) Cr(III) Cd(II)	Pb>Cr>Cd	Pb>Cr>Cd	Sulaymon et al. (2014)
Xanthate-modified magnetic chitosan	Pb(II) Cu(II) Zn(II)	Pb>Cu>Zn	Pb>Cu>Zn	Zhu et al. (2012)
Valonia tannin resin	Pb(II) Cu(II) Zn(II)	Pb>Cu>Zn	Pb>Cu>Zn	Şengil and Özacar (2009)
Eicchornia crassipes	Pb(II) Cd(II) Zn(II)	Pb>Cd≈Zn	Pb>Cd≈Zn	Mahamadi and Nharingo (2010)

Table 7. Selected quaternary sorption systems

Adsorbent	Solution component	Metal ions	${\mathit{q}}_{\mathit{m}}$ (mg/g)	${\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{X}}^{\mathbf{\prime }}/{\mathit{q}}_{\mathit{m}\mathbf{,}\mathit{X}}^{\mathit{o}}$	% ($\mathrm{\Delta }{\mathit{q}}_{\mathit{m}}$)	Reference
Mentha piperita carbon	Single component	Pb(II) Cu(II) Ni(II) Cd(II)	53.19			Ahmad and Haseeb (2017)
	Quaternion component ($C_o$ ratio 1:1:1:1)	Pb(II) (Pb/Cu/Ni/Cd)	50.50	0.949	−6.00%
Cabbage waste	Single component	Pb(II) Cu(II) Zn(II) Cd(II)	60.568 10.315 8.970 20.568			Hossain et al. (2014)
	Quaternion component ($C_o$ ratio 1:1:1:1)	Pb(II) (Cd/Cu/Zn/Pb) Cu(II) (Cd/Cu/Zn/Pb) Zn(II) (Cd/Cu/Zn/Pb) Cd(II) (Cd/Cu/Zn/Pb)	15.085 2.415 10.170 8.697	0.249 0.234 1.134 0.423	−75.1% -76.6% +13.4% -57.7%
Perlite	Single component	Pb(II) Cu(II) Ni(II) Cd(II)	26.9 5.66 3.35 2.81			Vijayaraghavan and Raja (2014)
	Quaternion component ($C_o$ ratio 1:1:1:1)	Pb(II) (Pb/Cu/Ni/Cd) Cu(II) (Pb/Cu/Ni/Cd) Ni(II) (Pb/Cu/Ni/Cd) Cd(II) (Pb/Cu/Ni/Cd)	22.8 3.80 1.64 1.34	0.848 0.671 0.490 0.477	−15.2% -32.9% -51.0% -52.3%
Coco-peat biomass	Single component	Pb(II) Cd(II) Cu(II) Ni(II)	100.3 17.0 24.3 10.6			Vijayaraghavan et al. (2016)
	Quaternion component ($C_o$ ratio 1:1:1:1)	Pb(II) (Pb/Cd/Cu/Ni) Cd(II) (Pb/Cd/Cu/Ni) Cu(II) (Pb/Cd/Cu/Ni) Ni(II) (Pb/Cd/Cu/Ni)	84.2 8.81 16.5 5.67	0.840 0.518 0.680 0.535	−16.1% -48.2% -32.1% -46.5%

Table 8. Major conclusions from selected multi-metal sorption studies

Biosorbent	Metal ions	Multi-metal adsorption model	Major findings/conclusions	Reference
Cabbage waste	Cu(II) Pb(II) Zn(II) Cd(II)	Competitive Langmuir Equation$q_{e,i}=q_{m,i}{b}_i{C}_{eq,i}{\left(1+{\sum }_{j=1}^n{b}_j{C}_{eq,J}\right)}^{-1}$were $q_{m,i}$ and $b_i$ are physical parameters, and $C_{eq,i}$ represent the equilibrium concentrations in the mixture of the sorbates.	Biosorbent more selective towards Pb(II) than to other 3 metals Pb(II) strongly inhibited adsorption of other metal ions	Hossain et al. (2014)
Soybean hull	Cd(II) Pb(II)	Competitive Langmuir Equation	There was competition, interaction or displacement effected between Cd(II) and Pb(II) ions	Módenes et al. (2015)
Pine cone shell	Cu(II) Pb(II)	Extended Sips isotherm$q_{e,i}=q_{mi}{b}_i{C_{ei}}^{\frac{1}{n_i}}{\left(1+{\sum }_{j=1}^N{b}_j{C_{ej}}^{\frac{1}{n_j}}\right)}^{-1}$where $q_{m,i}$ is the theoretical maximum biosorption capacity of metals “i” and “j” obtained by fitting of equation with experimental data for a multi-metallic system.	Biosorption capacity of Pb(II) was consistently greater than biosorption capacity of Cu(II) for both single component and binary sorption systems	Martín-Lara et al. (2016)
Volania tannin resin	Pb(II) Cu(II) Zn(II)	Competitive effect deduced from plot of $q_{e,i}$ Vs $t$	Biosorption capacities of metal ions lowered by 18–80% of those for single metal systems Pb(II) ions preferentially adsorbed compared to Cu(II) and Zn(II)	Şengil and Özacar (2009)
Calcium alginate, algal biomass, algal/glutaraldehyde-crosslinked polyethyleneimine (PEI)	Pb(II) Cu(II)	Competitive Langmuir Equation	The three sorbents showed a greater affinity for Pb(II) compared to Cu(II) regardless of the molar ratio of competitor ions	Wang et al. (2017)
		Langmuir-type model $q_{M1}=\frac{\left(q_m/K_1\right)C_f\left[M_1\right]}{\left\{1+\left(1/K_1\right)C_f\left[M_1\right]+\left(1/K_2\right)C_f\left[M_2\right]\right\}}$	Pb/Cu system For equal residuals of Pb(II) and Cu(II) close to 97% of total metal uptake was due to Pb(II) Pb/Cd system For the same residuals of Pb(II) and Cd(II), Pb(II) amounted to 88% of the total uptake Pb/Zn system For equal residuals, 99% of the total uptake was due to Pb(II)	Hammaini et al. (2002)
Arthrospira (Spirulina) platensis and Chlorella vulgaris	Pb(II) Zn(II) Ni(II)	Competitive Langmuir model	Whereas Pb(II) significantly reduced the adsorption capacities of the Ni(II) and Zn(II), the effect of Ni(II) and Zn(II) on Pb(II) was almost negligible.	Rodrigues et al. (2012)
Myriophyllum spicatum.	Pb(II) Cu(II) Zn(II)	Competitive Langmuir model	The biosorbent had no preference for Cu(II) and Pb(II) in spite of their differences in physicochemical properties. Effect of Cu(II) and Zn(II) on Pb(II) adsorption were similar. Explained in terms of similarity in molecular mass (63.57 and 65.38 g/mol), ionic radius (73 and 74 pm), and electronegativity (1.90 and 1.65 Pauling).	Yan et al. (2010)
Green macroalga, Caulerpa lentillifera	Pb(II) Cu(II) Cd(II)	Partial competitive binary isotherm model $q_{\left[M\right]}=\frac{q_m\left(b_M+b_N{b}_{NM}{C}_e\left[N\right]\right)\left(C_e\left[M\right]\right)}{1+b_M{C}_e\left[M\right]+b_N{C}_e\left[N\right]+\left(b_M{b}_{MN}+b_N{b}_{NM}\right)\left(C_e\left[M\right]\right)\left(C_e\left[N\right]\right)}$	Adsorption capacity was always reduced in the presence of a secondary metal ion. Adsorption preference followed the order: Pb(II)>Cu(II)>Cd(II) Presence of Pb(II) more significantly decreased the sorption of Cu(II) and Cd(II) than vice versa.	Apiratikul and Pavasant (2006)
Eichhornia crassipes	Pb(II) Cd(II) Zn(II)	Competitive Langmuir isotherm model	Combined action of the metal ions found to be antagonistic Metal sorption followed order: Pb(II)≫Cd(II)>Zn(II) Competitive model not applicable to Zn/Cd system	Mahamadi and Nharingo (2010)
Marine bacterium Pseudoalteromonas sp. SCSE709-6	Pb(II) Cd(II) Zn(II)	Competitive Langmuir isotherm model Partial competitive Langmuir	Lead highly dominated sorption of the other metal ions and adsorption capacity followed the order: Pb(II)>Cd>Zn(II); No competition existed between Pb and the other two metal ions (Cd and Zn); Competitive adsorption confirmed for the Cd/Zn system	Jiang et al. (2017)
Chitosan based nano-composite	Pb(II) Cd(II)	Competitive Langmuir isotherm model	For the same operating conditions, the composite showed higher adsorption of Pb(II) than Cd(II). Similarly, decrease in adsorption of Cd(II) in the presence of Pb(II) was more than decrease in Pb(II) adsorption in the presence of Cd(II).	Maity and Ray (2018)
Industrial chili seeds (Capsicum annuum) waste	Cd(II) Pb(II)	Competitive Langmuir isotherm model	The affinity of Pb(II) for C. annuum was more than 5 times higher than that of Cd(II).	Medellin-Castillo et al. (2017)
Xanthate-modified magnetic chitosan	Pb(II) Cu(II) Zn(II)	Original Langmuir isotherm equation	Combined action of the metals found to be antagonistic Metal sorption followed order: Pb(II)>Cu(II)>Zn(II)	Zhu et al. (2012)
Alginate-immobilized Trichoderma	Pb(II) Cu(II) Cd(II) Zn(II)	Not mentioned	Pb(II) removed more efficiently in multi-metal system Adsorption order followed: Pb(II)>Cu(II)>Cd(II)>Zn	Chew and Ting (2016)
Exiguobacterium sp.	Pb(II) Cd(II) Zn(II)	Not mentioned	Adsorption order followed: Pb(II)>Cd(II)>Zn(II)	Park and Chon (2016)

Table 9. Major conclusions from selected studies applying Response Surface Methodology

Adsorbent	Metal ions	Surface Response Methodology	Summary of conclusions	Reference
Aspergillus niger	Pb(II) Cd(II) Ni(II)	Central composite design (CCD) and design of experiment method (DOE)	Adsorption capacities for the removal of metal ions followed the order: Pb(II) (4.7 mg/g)>Cd (2.2 mg/g)>Ni (1.6 mg/g).	Amini and Younesi (2009)
Immobilized spent Tricholoma lobayense	Pb(II) Cu(II) Cd(II)	Central composite design followed by mixture design	Preference of biosorbent for the three metals in ternary sorption was in the order of Pb(II)>Cu(II)>Cd(II). Lead ions could still be effectively removed from aqueous solution in the presence of both cadmium and copper ($q_{max}$ = 370.4 mg/g), while removal of the cadmium and copper ions would be suppressed by lead ($q_{max}$ = 22.1 and 91.7 mg/g, respectively).	Cao, Liu, Cheng, Jing, and Xu (2010)
Aluminate treated Casuarina equisetifolia leaves	Cu(II) Pb(II) Ni(II)	Central composite design (CCD)	Single metal adsorption capacities followed the order: Cu(II) (44.0 mg/g)>Ni(II) (39.0 mg/g)>Pb(II) (28.0 mg/g). However, for multi-metal systems, the order was: Pb(II) (21.7 mg/g)>Cu(II) 13.0 mg/g)>Ni(II) (7.8 mg/g).	Rao and Khatoon (2017)
Enterobacter sp. J1	Pb(II) Cu(II) Cd(II)	Combinative experimental design applying mixture design and response surface methodology	Preference of metal sorption of Enterobacter sp. J1 in single metal systems decreased in the order of Pb(II)> Cu(II)> Cd(II).	Lu, Kao, Shi, and Chang (2008)
Trichoderma viride	Cu(II) Cd(II) Pb(II)	Box–Behnken design	Biosorbent preference for metal sorption decreased in the order of Cu(II) (0.360 mg/g)>Cd(II) (0.139 mg/g)>Pb(II) (0.103 mg/g).	Singh et al. (2010)
Live yeast Yarrowia lipolytica 70,562	Cu(II) Pb(II) Hg(II)	Central composite design (CCD)	Biosorbent preference for metal sorption decreased in the order of Cu(II) (204.1 mg/g)>Pb(II) (188.7 mg/g)>Hg(II) (181.8 mg/g).	Alipanahpour Dil et al. (2017)
Pretreated sporopollenin	Pb(II) Cu(II)	Box–Behnken design	Order of maximum biosorption capacity was Pb(II) (6.10 mg/g)>Cu(II) (4.84 mg/g).	Şener, Reddy, and Kayan (2014)
magnetic Prussian blue nano-sorbent	Pb(II) Ni(II) Cu(II) Co(II)	Central Composite Rotatable Design	Adsorption capacities followed the order: Pb(II) (778 mg/g)>Ni(II) (155 mg/g)>Cu(II) (138 mg/g)>Co(II) (111 mg/g).	Uogintė, Lujanienė, and Mažeika (2019)
Grape stalks wastes	Pb(II) Cu(II) Cd(II) Ni(II)	Homogeneous Surface Diffusion Model (HSDM)	Values of the Langmuir affinity constant, b, followed the order: Pb (54.5 ± 0.2) ≫ Cu (15.2 ± 0.3) ≫ Cd (9.4 ± 0.1) > Ni (8.1 ± 0.2) for sorption in ternary mixtures.	Escudero-Oñate, Poch, and Villaescusa (2017)
Activated Carbon	Pb(II) Cd(II) Ni(II)	Central composite design (CCD)	Adsorption capacity sequence toward the metal ions followed the order: Pb(II) (9.44 mg/g)> Cd(II) (9.37 mg/g)>Ni(II) (4.52 mg/g).	Kavand, Soleimani, Kaghazchi, and Asasian (2016)

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